There are many ways to contribute to Apertium, from sending us lists of words or phrases you find that are incorrectly translated, to getting involved in creating a new language pair or programming on tools or user interfaces.
Regardless of the kind of contribution you want to do, the two things to start with are to subscribe to the mailing list apertium-stuff, which is where most of the discussion goes on. Also, come and idle on the IRC channel
To decide what you want to contribute, take a look at Development and Projects for some ideas we've had around programming, extending the engine, and have a look at the Incubator if you're interested in linguistic issues. If you can't find anything that interests you or piques your interest, just send an email or ask someone on irc and they'll be happy to help.
 Adding/fixing unknown words
If you have some words that are unknown in a certain language pair, you can help out by simply writing list of words and their translations, e.g.
house; noun; casa; noun f dog; noun; perro; noun m
into a file, and sending that to the mailing list. Most likely you want to send to the one called "apertium-stuff"; subscribe here, then attach the file and send it to firstname.lastname@example.org.
You can also send a spreadsheet file, if you prefer that.
 Contribute language knowledge
The Indirect contribution guide has some tips on how to contribute your knowledge of a language to create resources that we use in Apertium, such as
- Writing contrastive analyses
- Cataloguing resources
- Hand-translating text
- Converting dictionaries
- Contributing to related projects
 Getting more involved
Next you should install apertium, lttoolbox and some language pair to play around with.
If you want to create or contribute to a language pair, go through the New language pair HOWTO. This is required reading for anyone who wants to get involved with developing Apertium language pairs. Also take a look at Contributing to an existing pair, meant for those who want to contribute to existing language pairs. You can improve the quality of the translation for an existing pair by correcting errors in the dictionaries. You will find some hints at the page Finding_errors_in_dictionaries.
If you're a student, Google Summer of Code (or Google Code-In for high-school students) is a good way to get involved with Apertium, and the ideas page there has lots of project tips if you're more interested in programming than linguistics/language pairs.