So I'm using an Arduino to control an 8v circuit and I want to use an 8v PSU, I was wondering were I should plug the negative in. I was thinking that is should be the psu and Arduino but I'm not very sure. I am new to all of this stuff so, if what I am saying doesn't make sense lmk.
You may want to give this a read. Everything in your image should share a common ground. Next your "some transistor" is not showing a base resistor? Less knowing your transistor part number your drawing makes absolutely no sense at all. How about you explain your project in detail and explain the objective?
Ron
Thanks for your feedback I read through some of the form you sent, again sorry I'm quite new to this stuff and I dont have an app to make schematics, also I don't need to power 8v to power the light I actually meant 6v. Does this drawing make anymore sense? Is the ground in the right spot and will the light receive the 6v if I put pin 7 on high? Thanks
Not everyone has software to draw schematics which is why the forum tells people even a hand drawn image is fine. Every new enthusiast starts somewhere. 
Now your drawing makes much more sense. Your transistor a TIP120 is a Darlington (nice way of saying two transistors in one house). Depending on your lamp load current you could likely get by with a 1 K Ohm base resistor but try the 2.2 K as pictured. Make sure you get the pinout on your transistor correct. Also using the TIP 120 you may want to give this a read. No shortage of TIP120 Arduino circuits out there. A good example of what we call low side switching.
Not sure how you are powering your Arduino?
Ron
I think I have my way of powering this figured out. This is all suppose to be plugged in so, Would I be able to use a 9v psu to power the Arduino mega through the battery port connection? Other than that I know what to do. Thanks for the help!
The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may become unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts.
Yes you can apply an external 9 VDC using the DC connector.
Ron
How much current does the "light that needs 6V" demand? Previously, the "thing that needs 8V".
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