HTC mail is the mail app that came pre-installed on the phone. Under applications, it's called MAIL.
One of the quickest ways to clear cache on EVO is to download Android assistant and then click on tools and cache cleaner.
As for the Factory Reset, you can Google it, Wikipedia it, call the Sprint store, etc. It's been awhile since I've done it and wouldn't want to skip a step, but as I recall it was REALLY easy. You'll have to re-install all of your apps because the reset takes you back to the day you got the phone. As for your SD card, back it up to your PC.