1. Ask Good Questions
Firstly, ask good questions. Good and thorough questions are more likely to be upvoted to increase your reputation. When you ask questions you learn to, so you can help other people on the site with similar problems.
2. Edit Tags
Secondly, edit tags. This is something that anyone can do with access to google and some time on thier hands. Adding descriptions to tags that don't have descriptions already will help you further understand Ubuntu and help other pepole to learn about terms that they don't already know. You can edit tags by clicking on the "Tags" link in the top navigation bar.
3. Leave Helpful Comments
Thirdly, leave thought-provoking and helpful comments. Don't be so critical to the point of being annoying. Leave a comment if something needs to be clarified, this will help the asker of the question so that the people who are trying to answer further understand the problem. Don't leave comments saying that the asker did a good job on thier question, instead, upvote.
4. Be an active user
Fourth, be an active user of the site. You get badges for leaving so many comments, upvoting so many times, visiting the site for so many days in a row, etc. For more information on what badges you can get for doing what, go to the "Badges" tab in the top bar. Read other people's questions and maybe they will help you learn something.
You earn privlieges for gaining rep. The more rep you have, the more stuff you can do. Remember that the FAQ says that "reputation is a measure of how much the community trusts you". In real life, building up trust takes time. Gaining a high reputation is going to take a lot of time and effort. Take some time and read through the whead the FAQ for some more ideas on how to gain rep and use the site appropriately.