Look at the second "sticky" thread (right at the top) in this forum titled "Clearing you Cache". It will walk you through the process of clearing it through the recovery menu. Sometimes pushing the buttons exactly at the same time can be tricky, just keep trying until the menu comes up. It's not that hard. If I can do it, anyone can. I have used an app called Clean Master in the past, but I don't think it did nearly as good of a job as doing it through the recovery menu. Hopefully this will help.
BTW, make sure you select Clear Cache, not Reset unless you really want to do a FDR. Start with clearing the cache first.