Using Random Sequence as Key

Hello, I was wondering if noone can help me maybe someone could at least
give me some directions where to look for info or where to ask:
I wanted to index a table by a random key. Exemplifying, when a insert is
made, the id value is automatically filled with a random number between
10000 and 99999. I'm pretty new in pgsql so I could use a howto or
something, and I repeat, if you don't know how, directions to where I can
find info on that or where I can ask someone that might know would be
enough.

Thank you