To mine using a pool, you need to first register an account on a pool site.

Here are a list of pools:
https://en.bitcoin.it/wiki/Comparison_of_mining_pools

I use slush's.  The site you choose will determine the hostname, port, username, and password.  You can then run the software with those values to begin mining.

You should know there is a minimum withdraw amount from most (all???) pools.  Because of this, it is likely you will need to mine for a long time with the current difficulty to withdraw any profits.