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 pool
http://mining.bitcoin.cz/accounts/register/

4) If you haven't, register a new worker on slush's pool
http://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 card
https://en.bitcoin.it/wiki/Mining_Hardware_Comparison

Just type it into the Extra Flags field like this:
-w128

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.