Like Thunder Cactus said start with a good hop up rubber. It's a good idea to get a good hopup assembly as well. They are one of the cheaper pieces you can buy. It'll help with seal and barrel wobble. Then work backwards into your air nozzle piston, the down towards reinforcing the gear box. Get yourself a solid motor, and a good battery to turn it. Once your internals are solid and reliable look to your spring and barrel. Tightbore won't always help. If you BB's are stable and straight don't change your barrel. Increase your spring to aim for 390 - 400 FPS with 0.2's, but run 0.28's in game. Always use a good qualty BB always.
But a big thing you could do is buy a "reliable out of the box", weapon to begin with. Classic Army, King Arms, ...etc. This way you can take your time and upgrade as you want. By the time you fully upgrade a cheaper gun you will end up spending as much or more anyways. And your gun won't break down on you often with a better quality starter.
ďREALITY IS LIKE A STONE. TO MANY ITíS HARD AND COLD, THEY CANíT HUG IT OR EAT IT, IT ONLY FRUSTERATES THEM AND DOESNíT DO THEM MUCH GOOD. TO OTHERS ITíS STRONG AND DEPENDABLE, YOU CAN BUILD WITH IT, BUILD UPON IT, OR WORK WITH IT, ALSO USE IT TO SMASH PEOPLE IN THE FACE.Ē