Giving an actual answer to this is difficult. There are many things you need to do to be a good Soldier.
In my opinion, some of the most important things are aiming (obviously), positioning, and target priority. If you understand who on the enemy team you should be focusing, while positioning yourself well enough to not just die, you're doing a good job. But again, there's a lot more to it, especially when you rise in the ranks.
The easiest way for me to give you feedback on what you should be doing differently as Soldier is either spectate you or see some of your recorded (recent) gameplay as him.