Best thing you can do is find what works best for YOU, not what works for others.
Here's a good guide
Step 1:
Join all of eGO's servers, play on them for a bit of time, and find which one you like best. Then stick with that server for a while so you learn the map well. No matter how good you are shooting, you need to be tactical, and know the map very well.
Step 2:
Play with ALL of the guns. Find which gun you like best. I've seen players get 70 kills with a M2 shotgun, and 3 with an AK47. Each gun will reflect a certain style. Guns like the Galil and P90 work great for close range "run and gun" people. I personally am a more "hold the trigger until they die" person, so a gun like the AK47 isn't for me, because it will just end up spraying all over. But a gun like the P90 has a tight spray pattern.
Step 3:
Learn your gun. Learn its spray patterns, its recoil, the time it takes to pull it out and start shooting, and how fast you run with it compared to holding a knife (because if you didn't know, you run slower with almost every gun than you do with a knife, and speed is sometimes helpful) Also learn reload times, and how many shots it takes to kill. Learn where to aim, and learn where not to aim. Because not every gun works with the "point at head and get headshot" method. I can get a para headshot aiming down at someones knees, but that's not the same as getting one with an AK47. A good way to practice with your gun is against bots on easy.
Step 4:
Setup your crosshairs. If you took the time to learn your gun properly, you wont need your crosshairs constantly expanding/contracting to tell you your spray pattern. Most "pros" that I have talked to get rid of the crosshair movement, and make the crosshairs as small as you can. I personally have mine set as a medium sized yellow dot, no crosshair lines, just a dot. Someone recommended me that, and the moment I changed my crosshairs, my score drastically got better. So you'd be surprised at the powers of the crosshairs
Step 5:
Be a team player, and don't be stupid. It may seem funny to go rambo running through a smoke gernade into a mob of 5 alone, but it's no way to get a good score. If your whole team is going one way, don't go the other way. If you stick together, you'll be less of a target, because there will be more of you for the opponent to aim at. But if you go solo, you'll have multiple guns aimed at your heads. Stay smart.
Step 6:
Spec the others. Truth is, your going to die a lot trying to get good. But in between rounds, spec the others. Look at who's at the top of the leader boards, and watch what he/she does. It may not be best to copy them exactly, but pull bits and pieces from everyone. There are commonalities you'll notice between the good players. And it never hurts to steam chat them and ask for a few pointers.
Step 7:
Get out there, play, and have fun. Bad moods=Bad scores. Joke around on mic in between rounds, dont play tired, laugh at your mistakes, and learn from them. If your mood is good, others follow, and when everyone on your team gets upbeat, they help you out, and make your kills easier.
That is about as far as I'll go for now, feel free to add on if you'd like. There's really no one way to get good, but I left you (O.P.) a few things that may help guide your choices.
-DOOM