There are many, many ways to contribute to Ubuntu. Some require technical knowledge, but many do not.
Before contributing to Ubuntu, you should read the Ubuntu Code of Conduct, which lays out a standard of behavior for contributors.
(Mostly) Non-technical:
Perhaps the easiest way to contribute to the Ubuntu community is to provide support. Besides helping out friends and family, you can answer questions on IRC, the Ubuntu Forums, or on this exchange!
A new project that is looking for contributors is the Ubuntu Manual project. According to their website,
Getting Started with Ubuntu is a complete beginner's manual for
Ubuntu, featuring comprehensive guides, How Tos, and information on
anything you need to know after first installing Ubuntu.
Their Get Involved page says they are looking for authors, editors, translators, programmers, and designers -- there's something for everyone!
Another project for writers is the Documentation Team. They write the documentation that comes with Ubuntu and work on the help wiki.
The One Hundred Papercuts project calls for users to submit bug reports on small usability problems (paper cuts). This requires almost no technical knowledge and has a direct affect on the Ubuntu experience.
Ubuntu strives to look good, and it takes artists to make that happen. Submitting art is an easy way to make a visual impact on Ubuntu. Check out the Artwork wiki page for information about that.
A really fun way to get involved is to join and participate in your Local Ubuntu Community (LoCo). You can check out the list of Ubuntu LoCos to see if there is an active community near you. If there is none, you can start one yourself!
As Javier Rivera commented below, multilingual users should look at contributing translations. Someone new to translations can get started with the Translations Quick Start Guide.
More Technical:
If you're the programming sort, there's plenty of room for you to pitch in. I'm not an Ubuntu developer myself, but the Developers page on the Ubuntu website is a good place to get started.
At the beginning of a new project called Operation Cleansweep, there were over two thousand potential bug fixes on Launchpad just waiting to be applied. This project works to review those patches and get them applied to their projects.
Conclusion:
There are innumerable ways to contribute. These are only just a few! Technical knowledge is helpful, but not absolutely necessary.
Some places you can go when looking for help when using the RTFM method ...
The Official Ubuntu Documentation page where you can find the official documentation developed and maintained by the Ubuntu Documentation Project.
The Ubuntu Documentation Wiki, whose home page I can never seem to find with a search, is a community-driven and open area of the Ubuntu documentation website which anyone can edit.
The Ubuntu Wiki. (I'm never really sure what this site is about or where to look for things on it, but there it is all the same. It does have a somewhat pretty page containing Ubuntu Release Information)
Best Answer
Visit the Ubuntu Wishlist Wiki or Ubuntu Brainstorm and suggest away!!