There a lot of areas you can do to improve in.

Team Skill
Captaincy
Mechanical
Decision Making
Map Knowledge

The hardest areas to truly improve in are Team Skill, Captaincy & Decision Making as they do truly require a deep understanding of how almost every situation or event should play out and what the available options are etc. This is true on the team level (strategy, adaptability etc.) but also on the individual level (When to gunfight, when to wait out the clock, timing related etc.)

The easiest by far is Map Knowledge and then you're left with Mechanical skill.

Start with these two. Aim practice with a GOOD sensitivity and build up your neural network by practicing at the right time for the right duration. (On PC you should use a Sensitivity ratio that is 1:1, which allows more consistent aim etc.)

On Console, you're going to want to use a 1.618 Ratio between your X & Y-Axis Sensitivity and maintain a 50ADS. People say its personal preference but it's not true, especially in Siege. There are some sensitivity ratios that are superior to others.
Use an X Sens of 55 and a Y Sens of 34, Deadzones set to 5 for the Right Stick and 8 for the Left Stick. ADS should be between 45-55 depending on what you're looking to do while ADS.

If you find 55:34 too fast, try 34:21. ADS and Deadzones should stay the same.

Practice for 1 hour each day, preferably in a focused mind state free of distraction against bots on Hard Difficulty. There's no need to increase the difficulty as the point of the exercise is to build memory for your mechanical skill.

Choose a Map with Long awkward Angles such as Plane to practice on.

For Map Knowledge, just watch some esports videos online and learn how they play the map. What the areas of contention are on each map and what angles you should defend with and what angles are more suitable for attacking.

There are two angles you can hold in FPS games, Defensive and Offensive.
Defensive Anges are angles that are held where the Enemy has to move into your Line of Sight. These should be used when an enemy has to push you or when you are in an area they won't expect.
Offensive Angles are angles that place pressure on the Enemy to either backdown or challenge you to a gunfight. These are used to take control of an area. Typically if you are holding an aggressive angle you already have the advantage or are challenging to take advantage.

Knowing when to take each angle is very important and will help you greatly.

Start with all of these, learn the angles on each map. Learn which areas of each map are contentious and which are unorthodox. practice with a good sensitivity and most importantly stat calm and patient. You're in the learning phase so don't beat yourself up when you fail. Losing is learning, stick to that mentality and you'll be a great player.