Hi, How should someone choose a transistor for a project? I know my current and voltage requirements. I want to control 1A and 12V with my 5V Arduino(or ATTiny), The problem is I don't have much availability, and i should choose from what's available which is still a huge list to check every single one! for example I couldn't find tip120(I needed SMD anyway, but chose a 2n2222 because I had already heard about it and used it!)
I use the digikey catalog to find new parts. It's got incredible sorting features (and digikey is where you probably want to buy from if you're in the US - first class mail shipping is dirt cheap and lightning fast. You need to make sure you get a logic level fet.
Without any other parameters, knowing what you've shared... a DMG2302 or SI2302 would by my go-to part, or SI2312CDS if you want to overbuild it. All SOT-23 packages, and will work even if you later decide to run the electronics at lower voltage. They're what I sell on breakout boards, and what I use in my own projects (except where higher current is involved, ofc)
kamhagh:
I want to control 1A and 12V ......chose a 2n2222 because I had already heard about it and used it!)
A 2N2222 definitely isn't suitable. It has an absolute maximum of 0.8A, meaning you should stay comfortably below that figure.
I'm with DrAzzy, use a MOSFET. I assume that's a logic-level device. (Or he wouldn't have recommended it.)
What about a FDN335N?
I searched for this:
These are a few of the popular transistors for high gain NPN in small signal low voltage applications Popular= common and low cost.
High gain isn't appropriate for switching applications, good saturation properties are
the thing to look for - switching is a different regime from analog amplification.
The best small switching transistor I've found is the ZTX851, nothing else comes close
in a TO92 sized package. However as has been pointed out go for a logic level MOSFET,
certainly once you are in the amp domain rather than milliamps.
DrAzzy:
I use the digikey catalog to find new parts. It's got incredible sorting features (and digikey is where you probably want to buy from if you're in the US - first class mail shipping is dirt cheap and lightning fast. You need to make sure you get a logic level fet.Without any other parameters, knowing what you've shared... a DMG2302 or SI2302 would by my go-to part, or SI2312CDS if you want to overbuild it. All SOT-23 packages, and will work even if you later decide to run the electronics at lower voltage. They're what I sell on breakout boards, and what I use in my own projects (except where higher current is involved, ofc)
Thanks 
I don't live in us, I live in middle east(I don't think I need to explain more!) and you can't find any good stuff(electronics are the same) But that website will probably help me with my future choosing! and I was wrong, my motor is 0.8A which is just about the voltage my motor needs! but it will be just on for like once a day for like 20seconds! I can add a resistor however, I don't need the power my pump has!
0.8A at rated load or 0.8A stall?
MarkT:
0.8A at rated load or 0.8A stall?
not sure, not much info on the website! i'll use a resistor anyway and my printed pcb doesn't work so i guess it doesn't matter I'll get a safer transistor if i make another board! It's weird every connection sounds fine, yet programmer is out of sync only when connected to it, already made a thread and soudns like i fried my AtTiny13a by droping a bit of hot solder on it!
I can't get to Digikey btw, sounds like it's forbidden for my country!
and i couldn't find any of the mentioned mosfets/transistors in here ! I didn't look for a mosfet as i think my arduino is not powerful enough!
kamhagh:
i'll use a resistor anyway
For what?
to limit current of my motors!. but as my current board doesn't work there's no point using the same transistor in my next one
At 5 volts your resistor would need to be a 4 watt resistor you don't need a resistor and most times you don't need a MOSFET And i bet you can find a good one for free. Find a old radio controlled car. And you'll have about 8 of them for free. There used for the h-bridge in the car.
The power transistors in this all came from a broke car I got for free with heat sink it's could handle 3 amps easy I'm using it on 2 motors that stall at a 1 1/2 amps.
be80be:
At 5 volts your resistor would need to be a 4 watt resistor you don't need a resistor and most times you don't need a MOSFET And i bet you can find a good one for free. Find a old radio controlled car. And you'll have about 8 of them for free. There used for the h-bridge in the car.
The power transistors in this all came from a broke car I got for free with heat sink it's could handle 3 amps easy I'm using it on 2 motors that stall at a 1 1/2 amps.
I think i threw them all out but i'll check sounds good! as I'm making a robot soon 
also it will help with my pump if i decide to make another (which i guess i probably will)