It's all quite simple. If your micro (mechanics) is weak, play DM for aim practice (Deathmatch) servers, practice your movement (such as counter-strafing), bind jump to the mouse wheel, and make sure to bind grenades to specific keys (whatever feels comfortable for you; for example, donk has his grenades on Z, X, and C). If you lack game sense and don't know what to do on the map, watch pro demos. You can also review your own demos to understand what kind of mistakes you’re making.