Please help me with a few basic things…

I'm still very much a beginner and I have ADHD so that gives me an extra layer of challenge. Please help me if you can.

1. I don't quite understand when se and te are used as opposed to tu/usted?? When/where are they used?
2. Any tips on how to remember to use female vs male versions of a word? Any tips on how to remember whether a word IS feminine or masculine? I often have trouble remembering la/el though I have no trouble remembering contento/contenta for instance, just have trouble remembering to use one rather than the other.
3. When do you use which happy? lol I mean alegrar vs feliz vs contentar? Examples? I do best with examples.
4. Thanks in advance for any responses. I appreciate it!