The way I learned was to jump in one and just play.
I was quite nervous my first time too - I didn't know what to expect or how to even really play.
The mod helps because he/she will explain your role and what your power is.
Mikeburnfire site helps a lot too.
You guys will catch on quickly.
The better BS artist you are, the better you will be at mafia.
Agree with NM - it is a fun time, even if you die.
Simple games are good to start on.
When a theme and alternate roles get thrown in it can be a bit too much to take if you aren't used to it.