Most of the time, everything begin by a mailing list. No exception here, you can join the documentation project by subscribing following this link http://wiki.documentfoundation.org/Documentation/Development.

On that page, you'll find information on the Guides that are worked in English. Always up to date and maintained by a team, some of them still need some love however. It's a very huge work to keep them reflecting the suite enhancements, versions after versions. But Jean Weber is doing a fantastic work here, coordinating and keeping every body in the go. If you don't feel comfortable with writing, you can be a proof reader. Proof reading means that you check for the language, but also for the accuracy of the description to implement a functionality or a task. You see, you can learn a lot through proof reading. If you would like to participate in the writing, just pick a chapter, inform the list and go on. There are documents on the wiki that will help you to begin with any tasks you want to achieve. And don't forget, you're part of a group, so other members will always assist you.

Along with the Guides, the FAQ is a very good tool for quick help and is often use for support on the mailing lists or forum. Writing a FAQ is quite easy: you'll find the question and answer on the list. But you know that a product evolves and so the information on how to use it. So somebody need to go through the previous articles and maintain them up to date. How does it feel for you? Have a look here: http://wiki.documentfoundation.org/Faq. Yes, a lot of red links... this is an ongoing translation of the French FAQ (this last one is maintained since SO 5.2 ;-) but translating is not an obligation, if you know how to fill the article, just go ahead! A documentation written in English will make it available for the overall community as English is our common language.
.

Speaking about translation, you'll find Documentation projects in several languages. Sometimes the content will be translations of the documentations produced in English, but sometimes it will be full creation. Process of learning are not the same everywhere, so no problem to adapt it and create what fits the best for you, everything is produced under open source licences. The only important thing is to work with others, let them review and criticize your work to improve it and make it better for all.

So what about other forms of documentation like videos, tutorials... You are more interested on that sort of participation, great, we need you here :-) Your ideas and realizations are welcome if you have any skills in crafting such material.
In our constant focus on lowering the difficulty to access to our product, we also maintain help files delivered along the product and copied over a wiki. This work is translated in lot of languages and it's a huge effort. This is a place where your work will have a lot of recognition from the developers and the users. Of course you don't need to know how to code or use the localization tools to do that, but we can organize a workflow where you notify the project of your changes, or we can grant you access to the wiki or... we can imagine whatever will be pleasant for you. To make it available for all, the primary language of this work is English. Document a new functionality, update the previous one that has been enhanced, you see, no time to loose :-) These help files are most of the time the only documentation available for small language communities, so it's an effort that helps the use of LibreOffice in lot of places.

The documentation project offers also several interactions with other projects like User Experience or Marketing or Localization. While you're visiting all the functionality of the suite, you'll see some dialogs that could be better designed, or that could be better labelled. Don't hesitate also to report your findings, we are always happy when we can improve and extend the usability of the suite. Even if you don't have the knowledge, sometimes it's a matter of common sense because you use this function every day when we don't. I spoke about Marketing, interactions will be by proof reading the material, try to work together on the common messages we want to convey. And Localization, because as for UX, you may find some wrong labels, or labels that could be better turned or even better translations if you work in another language. You see, we are a community, working together in a transverse manner.

And if you really don't want to write or to read what others have done, you can still be helpful by welcoming other contributors, let them know how they can join a work in progress. You can also help with the workflow between the wiki, the CMS, the site and even book publication or the communication with other projects. And if, finally, you still don't know what to do, let me know, I'm always full of resources to propose work to others ;-)