1. aim. aim isn't as important in siege but every day you play you will improve your aim in some way
2. map knowledge. most important thing in siege is map knowledge i.e. knowing how maps playout, what ops people like to play on each map. Callouts are an important skill; giving extremely precise calls is very important. Go on private server/t-hunt and just play around the maps
3. Attacking. Most difficult part of siege especially with the current meta. Know your role and play it well; most important ops are Thatcher and Thermite, if you choose to main/play them DO NOT get spawnpeeked. And drone, drone, drone. Drones are your most important tool on attack. If you lose both, you're playing blind.
4. Defending. Don't peek = win. Reinforce important spots and don't peek.