I'd like to contribute to Wikipedia. What should I do?
First, make a list of everything you know. Then, write an article for every item on that list. If, for some reason, this is not practical, here are some other tips to help you get started.
Choose a topic that is of interest to you. Find out what has been written about that topic so far. Make improvements to the articles that already exist. Fill in the gaps by creating new articles on aspects of the topic that have not been covered yet.
Visit a Random Page. Find a link to an article on that page that hasn't been written yet. These are the links that have little blue question marks after the words. Click on the question mark, and add something about that topic.
Write about something you don't know about. Use this as an excuse to research a new topic. As you learn about it, write what you are learning here on Wikipedia. This is actually a good study aid because it forces you to take notes, to organize information, and to put what you've learned into your own words.
Make a project! Take a book and write a thorough chapter-by-chapter summary. Make pages for all the characters. Or do this with your favorite fictional universe (Star Wars, Middle Earth). Pick your favorite actor and review all of their movies. Pick your favorite city and build pages for all its best attractions.
Find a big topic (like World War II or Chinese history) and create a main page, and all the subcategory pages. List all the topics that should be covered for a complete treatment. You don't have to write articles for all these topics. But if you build an outline and list everything that needs to be filled in, eventually someone will come along and start filling in!
You don't have to write to contribute. Edit pages for spelling and grammatical errors. Remove nonsense when you see it. Correct false or misleading claims. Wikify words (turn them into links). Find redundant pages like Chicago and Chicago Illinois and combine them.
Tell your friends, your enemies, random strangers on the street. Hire skywriters to write 'Wikipedia.com' across the sky. Get the word out.