1) We need to know what graphics card and operating system you are using.
2) If you're using windows, guiminer is probably easier:http://forum.bitcoin.org/index.php?topic=3878.0
3) If you haven't, Create a login on slush's poolhttp://mining.bitcoin.cz/accounts/register/
4) If you haven't, register a new worker on slush's poolhttp://mining.bitcoin.cz/accounts/profile/worker-edit/
The main login and worker login are not the same.
5) Use the worker login in the guiminer interface.
6) Check here for tweaks to your graphics cardhttps://en.bitcoin.it/wiki/Mining_Hardware_Comparison
Just type it into the Extra Flags field like this:
That should do it. I'd try to be a little more clear but I'm about to pass out.
If you need more help PM me or something and I'll try to help tomorrow.