Should probably start by buying a good rifle, marui L96, marui VSR-10 or CA M24
Any of the above only need around $700 of upgrades to make it shoot absolutely amazing
There's a lot to know about airsoft ballistics, lots of pages here about it, you'll need to understand all of it.
And you should probably start as a rifleman, with an M4. Nobody starts out as a sniper, not even snipers.