How to get familiar with a system

[Dieter BSD]

> i know my poor English

Some topics can be difficult even for native English speakers.

I assume you have already looked for books, magazines, web forums, etc.
in your preferred language.

> Maybe i really need to involve myself into a small, real, project,
> any suggestion?

There are plenty of problem reports that need fixing.  Some of the mailing
lists get a list of open PRs posted to them weekly, or you can search
with your web browser.
http://www.FreeBSD.org/cgi/query-pr-summary.cgi?query