Its quite easy.
A) Sign up for Slush's pool. I am not very sure on this process so I'll leave you to figure that part out.
B) Download the latest version of Kiv's Gui Miner https://github.com/Kiv/poclbm/downloads
(its 20120219 at time of writing)
C) Next you need to install openCl drivers for your make of GPU. It will be up to you to find these files mostly because you havn't even said what you are using.
D) You create a new miner in GUI-miner by first deciding which you want to use, OpenCL, then choose which server you want to connect to, Slush's for your case, fill in the info for logins and there you go.
There is a few optimization you can do with flags and such but being that I have no idea what cards you are running or if you are even GPU mining its hard to steer you in the right direction.
I hope this helps