Quote Originally Posted by xXSomeGuyXx View Post
1) How do I start?
Download a mod and install it.
2) It's legal, right?
Yes.
3) Do I need to learn any languages?
No. Almost all relevant mods are in English.
4) Can I make my own guns/characters/HUD?
Yes, but you won't be able to play in multiplayer with altered models and items.

For a good overview of the most popular online mods, check out
http://steampowered.com/v/index.php?area=stats&cc=DE

It will show you which mods actually have players.
I can personally recommend The Hidden: Source and Source Forts. A good SP mod is Minerva.