That depends on your operating system. If it is windows, then click on the following: Start (the Windows symbol in the lower, left-hand corner), All Programs (should be at the bottom), Accessories (also probably at the bottom of that list, next to a file folder icon), System Tools (also next to a file folder icon). Now you want to execute two programs consecutively. First, under the System Tolls rubric, you want to run disk cleanup. It won't be asking you to delete anything important, it will mostly just get rid of temporary files. It might delete cookies, but that's no big deal--your frequently visited web sites will give you new cookies if that happens. I just don't recall if that happens automatically.
After you have run disk cleanup, follow the steps back to System Tools, and select disk defragmentation. This takes a while, especially if you've never done it. This is cup of coffee and the morning paper stuff, the first time it's done it can take quite a long time.
If you have any problems . . . i was never here . . . i don't even know you. (Seriously, this is routine stuff, no muss, no fuss.)