I'm by no means the greatest aim, but I can hold my own against semi-pro players, players that typically compete for money in tournaments. I'm often accused of being a bot.
some tips:
1. Good equipment. I have 240fps monitor and wired gaming mouse. I thought I had it good with 60fps monitor but it makes a world of difference to have more.
2. Good settings. Your mouse sensitivity should be very low. I move my arm about 14 inches left/right to perform 180 rotation. It's a lot of movement and a bit of a workout, but it's the only way to make those flick headshots consistently. My mouse dps is set at 350, but really depends on your resolution also. My X resolution / mouse DPS = 4.8 (1680/350).
3. Good technique. Use your elbow and shoulder only for aiming. Do not use your wrist. Your wrist should always be straight.
4. Practice, of course! I play for about 20 minutes vs bots before I even go online.