Is it hard to setup 120v project with Arduino board
Do you mean that you want to control a 120V device ? What is the device ?
What is your level of expertise with mains voltage devices ?
For your safety and the safety of the Arduino and your computer (when your computer is connected to the Arudino) you MUST ELECTRICALLY ISOLATE the Arduino from the lethal voltages!
If you want to turn a 120VAC device on & off from the Arduino the easiest & safest solution is [u]this[/u].*
Or you can use a relay, which is an electrically-isolated, electrically-operated switch. The Arduino can't directly drive a regular electro-mechanical relay so you need a driver circuit, but you can buy relay boards with a built-in driver.
There are solid state relays that can be controlled directly from the Arduino. Industrial type solid state relays like [u]this[/u] are super-easy to wire-up with screw terminals (and easy to mount).
The coil/control specs for a relay are "exact". That is, if it has a 12V coil it must be controlled with 12V. The contact ratings are the maximum, so for example it's OK to use a 240V / 10A relay to switch 120V / 1A.
Solid state relays are a little more tricky on the switched side. AC & DC relays are not interchangeable and sometimes there is a minimum voltage rating (as well as maximum ratings).
- I assume it has a solid state relay inside.
JOJO954:
Is it hard to setup 120v project with Arduino board
It is easy to make it work. It is hard to make it work safely. You may harm yourself, someone using it as well as cause fire etc. If you are not sure what you are doing you should not attempt it without help.
Use relays to switch line voltages; don't touch the relay board while power is connected, don't work on the wires while power is connected. That should go a long way in not having yourself electrocuted and keeping things safe. Do remember to downrate cheap made-in-China relays by at least half.
Other than electrocution risk, it's quite easy to get sparks flying when you're working with mains voltages. At least it is at the 220-230V we have here; 120V is a bit harder but as your fuses are typically at double the amperage we have setting fire due to a short may be easier.
JOJO954:
Is it hard to setup 120v project with Arduino board
Please don't take this the wrong way because no insult is intended:
If you have to ask a question like this, you are not qualified to make any circuit that connects to the "mains" (120v or 240v).
These voltage levels can be and are lethal. High voltages rarely grant a second chance.
PLEASE consult with an electrical engineer or technician that knows how to safely handle these voltages.
A simple mistake can kill you. I'm serious! Be careful and get skilled assistance!
+1 to that! 
High voltages rarely grant a second chance.
I've had several "second chances"! 
The WORST shock I ever got was from a loose spark plug wire on a lawn mower. I don't know WHAT I was thinking but the motor was running rough and I could see the loose wire so I tried to re-attach it. The funny thing was that I was reluctant to touch the thing to re-attach it once the motor was stopped.
And once when I was in high school I got a pretty bad 120V shock - My mother said she was feeling a "tingle" from the waffle iron. I was wearing sneakers and I didn't feel anything but I knew a little about electricity so I guess I didn't believe her so I reached-over and touched the water faucet and the waffle iron at the same time... BLAM! I felt the current flow though my chest! Then I said, "That was stupid". I advised her to get rid of that thing, and I think she did...
And a couple of times I've had 120V across just my fingers, maybe current flowing between my thumb and finger. That's uncomfortable, but not too bad.
But, it's absolutely true that 120 or 240V CAN KILL YOU! So, I try to be careful and hopefully I won't need any more "second chances"!
DVDdoug:
I've had several "second chances"!But, it's absolutely true that 120 or 240V CAN KILL YOU! So, I try to be careful and hopefully I won't need any more "second chances"!
The worst shock I ever got was when I was 15 and working on a home made 40 meter transmitter.
The RF final was a 6146 running with 800 volts DC on the plate (a 65 watt transmitter).
Amazingly, I was granted a second chance... I accidentally contacted the 800V supply with my right hand while holding the transmitter chassis on it's side with my left hand.
I saw a bright flash of white in my peripheral vision (a reaction to the shock - there was no actual light). I felt like someone slammed me in the face with a wooden board (no pain, just the impact feeling and the "punched in the nose" smell).
The muscle reaction threw me off the chair I was sitting on. I was unconscious before I landed on my back on the floor.
I don't know how long I was out for, but when I woke up I ached all over, was very weak and my hands and legs were shaking.
I'm sure I damaged my heart because after the shock (and to this day) I have occasional PVC events (heart feels like it misses a beat, then the next beat is a strong "bump" in my chest like the heart is "making up for the missed beat").
I was a damn fool working on a high voltage circuit with the power on and something like 40uF of capacitors at 800 volts DC.
I am oh so lucky that I was granted my second chance... albeit with a heart arrythmia to forever remind me to give electricity the caution and respect it demands.
I'd speak to a cardiologist (if you can afford it, that is ) as to whether that shock was in any way likely to result in a tendency to ectopics. 
Remember that we use exactly that process (but in a more controlled manner) to actually correct arrhythmias with little or no permanent damage - although this is often in the context of prior damage to the heart.
I learned to be particularly cautious about the 6CM5 horizontal output anode cap of the first 21" TV we had which my father brought home sans cabinet and spent a lot of its time on its side on a card table while it was repaired for various quirks. Apparently this was only 300 V though (but the flyback pulse would have been much more).
In fact it is unlikely an electric shock will kill you. But when it manages to kill you chances for fast enough resuscitation are low - maybe you end "only" brain dead instead of completely dead.
Project involves me using 120v do I still need to use a 9 volt battery to power the servo motor
Could you take a few moments to Learn How To Use The Forum.
It will help you get the best out of the forum as a whole.
Bob.
I wouldn't normally power a servo off anything above about 7.5 volts.
JOJO954:
Project involves me using 120v do I still need to use a 9 volt battery to power the servo motor
Very unlikely. What servo? The specification for the particular servo will tell you what voltages it used but they're usually less than 9V and certainly less than 120V.
And if you mean a small rectangular 9V battery they're never of any use to power servos.
Steve
Its funny, I keep looking back at this thread hoping to see what the OP really has in mind. But in the 10 responses (now 11) no word from the OP. Perhaps we scared them off 
That would be too bad as I know we all started learning somewhere and understand new folks are still in that steep learning phase.
John
With the Arduino, you can easily control a ‘Power Switch Tail’ product, it then controls your 120VAC device.
Anything more than this, my guess, you are not ready to handle.
Paul__B:
I'd speak to a cardiologist (if you can afford it, that is
I have government employee health insurance (translation: probably the best and most complete insurance one can have without being a bottom feeding politician).
And I did consult with my doctor and two cardiologists. They all told me that most people have occasional PVCs and don't even know (feel) it.
The doctors are not sure if my nasty shock at age 15 has anything to do with the PVCs. I have no heart muscle damage that an X-ray, CT scan or blood tests can see.
I zapped myself when I was 15 and I'm 62 now, so I suppose if it was going to kill me it would have done so a LONG time ago...
Smajdalf:
In fact it is unlikely an electric shock will kill you. But when it manages to kill you chances for fast enough resuscitation are low - maybe you end "only" brain dead instead of completely dead.
Now that makes sense. My wife has stated many times that I "must be brain dead" to explain the latest stupid and/or dangerous thing I just did... 
JohnRob:
Its funny, I keep looking back at this thread hoping to see what the OP really has in mind. But in the 10 responses (now 11) no word from the OP. Perhaps we scared them offThat would be too bad as I know we all started learning somewhere and understand new folks are still in that steep learning phase.
John
Honestly, I would rather see the OP be "scared off" than have him try to connect PORTB to the AC mains.