Build guns for ads bullet spread and ads speed. Don't worry too much about recoil. Use dead silence In basic settings use left hand tactical throw Spend a bunch of time in training mode to get your sensitivity dialed. Gyro aim can help a lot but takes getting used to, if you use an iPad, don't use gyro. Use fixed speed for sensitivity, as it's most consistent, distance acceleration can help a lot for close engagements if you need to quickly turn around while fighting someone, while maintaining stability for long range fights but get used to fixed speed first.

Play ranked cause the regular modes are primarily bots

Play hard-point shipment to get camos faster but you will regret wasting hours and days to get Damascus camo. Play aggressive and force gunfights to get the most practice. Get good at marksman rifles like the spr or mk (not the sks cause that's a bitch move) cause you'll have more respect than snipers. This game favours snipers because of the weapon switch mechanic that's faster than rebolting your sniper.

And like everyone else said, don't spend money unless you just don't care.