So I've wanted to play PC for a long time and Im not sure where to start can anyone tell me what I would need to play Arma 3????
As far as the bare minimum requirements to run Arma 3, you basically want to lean heavy on the CPU(at least a 4-series intel chip like a i5/i7-4790k) and then 8gb ram is a minimum but i say that knowing that i used to play it all day and night on the old i7-3770k with little to no issues. The game is like 3 years old now so it should require too much.
That all being said, future proofing builds is a serious consideration and i would say to build with the best base components you can afford. Leaning heavily on the CPU socket thats not already a relic and a good motherboard. Everything else is easy to swap in new components when and if you would want to.
I personally can still run ARMA fairly well on my AMD 6300, I upgraded my GPU from a 650 to a 950 and saw a pretty good increase in frames in most of the popular cities, you need a top of the line PC to be able to run ARMA at 60fps which hasn't changed since GSN was involved with ARMA. Obviously though you need to build around the games that will be coming out in the future and not what would pass as a mid tier PC 3 years ago. So I can easily agree with Sams recommendation on parts.