As always, I recommend A&K M24. Clean it, use it. There is plenty of upgrade capacity in it for later. But yeah, if you can get an unmodified TM, that is the best.
I started with a Well MB06. Quite cr*p. I had L96 one or two, still quite bad. I have an upgraded Bar-10. This one is great. Both the Bar-10 and the A&K M24 take VSR parts (mostly) and that is still the best sniper platform.
What Mr Monkey Nuts said is very true. You have to be prepared to absolutely suck at the first dozen of games. On my first sniper game my only success was that I got my crosshairs on one guy (and missed the shot). I couldn't even aim at anyone else, and by that time I already had a few years of experience with my aeg. Ok, it was milsim, but that gives the idea how different sniping is to "aeging". On my second game I could shoot one guy, and then gradually I became better and better. I'm still not good.