If you want to participate in translating, you need to do a few things first. Basically, you need to:
Register at http://pootle.locamotion.org/register.html and wait for the activation e-mail. If you get a message saying “invalid user” or similar -- it is an annoying bug, so please let us know about it (and just try to log in again).
Once you've activated your account, log in using your username and password. Then visit http://pootle.locamotion.org/home/options.html, and select your first language and your second language. Optionally also select which projects you want to participate in (hold Ctrl to click multiple options).
The reason I need to know your username is so that I can give you translation rights. On many projects, anonymous or unknown users have suggestion rights, but no translation rights unless they are granted by me or by the language maintainer for that language and project.
We have a page for each project on our wiki, with useful information about how to translate it and where to find terminology for it.
For example, if you want to help translate FileZilla, visit http://translate.sourceforge.net/wiki/decathlon/filezilla/.
In many cases, it is necessary to register and make yourself known at the official page and forums of the project that is being translated. Such instructions will be on the above-mentioned wiki page. Try to appear like an active member, and you'll get lots of attention.