Figure out a problem and try solving it in bash - bash for beginners guide on tldp site can get you started. You get better as you use it.

http://www.tldp.org/LDP/Bash-Beginners-Guide/html/

EDIT : Additional links -

Advanced - http://tldp.org/LDP/abs/html/

Bash programming - http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html