There are 2 routes you can take.
1)Buy a gun that's got great internals out of the box(Krytac, Scorpion Evo, and some other rather expensive makes I won't mention)
2)Find a base gun you like and upgrade, either do it yourself or commission a tech to do it for you.
The most important things to do include making sure your barrel is clean and your using good quality BBs (0.25g at least, 0.28g are usually best) and making sure your hop up is set correctly.
If your using a sight or scope, make sure it's zeroed in!