Download the client to your PC from: bitcoin.org
Setup a bitcoin miner
ATI/AMD Users
You may need to update your video drivers or download and install the AMD Stream SDK from HERE before you can begin mining
Windows
Download GUIMiner: HERE
GUIMiner options
Server: Other Host: arsbitcoin.com Port: 8344 Username: <your user name>.<miner name> For example, BurningToad.default
Password: <your miner password (default miner password is 'default')>
You can view or edit your miner accounts in Account Details.
Device: Select the graphics card/cpu you would like to use
Extra Flags: Can be blank, but "-v -w128 -f 80" works well for ATI cards.
Linux
Check out Phoenix: forum post.