Find a pool you want to join. New pools are posted here regularly, and there's also an article on the wiki with a few:https://en.bitcoin.it/wiki/Pooled_mining
When you've joined a pool you'll have received a username, password, hostname and port number. Use the standard bitcoin program something like this to do work:
bitcoin --get --rpcuser=<user> --rpcpassword=<password> --rpcconnect=<hostname> --rpcport=<port>
or get poclbm or cpuminer or some other client that may be able to work more efficiently.