I am using an Arduino to control a solenoid that needs 2 A current DC but the Arduino only outputs 20 mA max. I have a bench top power supply that can supply 2 A but i need to find a way to use the Arduino to switch it on and off. I think BJT transistors might be the way to go but I am not sure which one i should use. I am open to any other suggestions but the simpler the better. A specific part no. and or circuit would be very helpful.
Do you really need an amplifier? Amplifiers are used when you want to amplifiy a signal that could be anywhere (from 0mA to 20mA). I suspect what you need is just a switch (seeing as the Arduino can't spit out an analog signal anyway - being a digital device and all). Paul's recommendation of an N-ch MOSFET is what you probably need or even a BJT transistor if you're feeling a bit more retro (the FET's have lower loss though).
Pretty much any BJT that's rated for the current and supply voltage will do provided that it has a current gain of >100 at 2A of collector current (otherwise it'll act more like a current limiter rather than a switch).
As for FETs, as long as the current rating and voltage rating are greater than what it'll encounter in your system, you'll be ok. You may just need to check the FET's "threshold voltage" (not Vgs_MAX, as this is usually +/- 20V or so) as this is basically the 'turn on voltage' i.e. the point at which it stops being an open circuit and starts acting like a closed switch, just make sure you grab a FET with a threshold voltage below 3V (the Arduino spits out 5V, but you don't want to be on that edge, 5V into a FET with a threshold of <3V should work pretty well)