I can't do a lot of things on here because I don't have a reputation. How do I get a reputation?
You get reputation1 for positive contributions to the site.
The most obvious way would be asking good questions and writing good answers - when other users consider your post helpful, they will upvote them.
Other contributions are also rewarded with reputation, e.g. suggesting good and helpful edits of existing posts. The details of how to gain and lose reputation can be found here in our Help Center.
1 Please note that “reputation”’on SE sites is a numeric value that reflects the quality of the posts etc. It’s part of the gamification mechanism on Stack Exchange. So while you want to work on gaining reputation (= points via upvotes etc.) you want to be careful how you behave as a member of the community lest you end up having a reputation.