solving A and B in div 2 is enough to become pupil. For that I will recommend you to solve problems of rating 1200-1300. Generally A is most of the time pretty straight forward and for B it require some little observation which you can practice on these 1200-1300 rating problem.

To be honest you are the only person who can know what you can improve. Reason for that is because there are many things which could be going wrong. Maybe you cant solve harder problems because you don't know how to think? Maybe you cant solve harder problems because you don't know needed math? Maybe you know how to solve harder problems but you stress out too much on contest? Maybe you have troubles with writing code or maybe troubles with debugging? The issue is that there are many thing which could be going wrong so nobody can give you the best advice.

Observe what are you doing wrong and try to see some patterns in that. After you see these error patterns then try to come up with how you can overcome this weaknesses. The answer may be for example to change how you practice, how you think or change your attitude. And of course, the most important thing, have fun.

Dont just solve easy problems.. after getting comfortable in a range move ahead. And rating will eventually increase try not to focus much on it.

1. There is a theory (or not just a theory) that you have to solve problems in [desired rating: desired rating + 200] range.
2. Whenever you train make sure to not deceive yourself with "solving" problems with editorial. You should use it (editorial) when it is indeed needed, not just 5 mins after you opened the problem.
3. Don't make excuses and make sure to face your drawbacks.

I recommend you to read this blog I read it about 4 months ago and it completely changed my practice strategy