When you first open CCleaner you will be presented with the following screen:
From here, you can customize your cleaning options by selecting or clearing the check boxes, or you can stick with the default settings. The options that look like they are greyed out can be selected, but are not done so by default.
When you have decided what you want to clean just click Analyze. This will allow you to see how many junk files are currently taking up unnecessary space on your computer. The analysis should only take a few seconds, and will display a list of applications that are taking up space with details of the files responsible.
If you want CCleaner to remove these files and clean the specified areas, click Run Cleaner. Afterwards, you will see a summary of the applications cleaned. Double-click a specific application's name in the Cleaning Complete list to see exactly which files were removed.