The key is to play the game a lot, think about the game a lot, and watch others play the game a lot. Make sure you're running private servers by yourself to check out each of the maps, and get comfortable with the pros and cons of each weapon and item.
Try and find more casual servers when you start, and as your reflexes and tactical thinking increases, try and move on to servers that are more competitive.
It's really generic advice, but that's because it's the same for many games. It's really about practice and pushing yourself to go that extra mile. The people who build a great body shape have to work extremely hard to get as good as they are, and the professional CS players have been playing for years.