Q: how do I speed up/clean out my mac- it's really slow!

"My macbook is about 8 years old...and has been upgraded to Yosemite..."

So this is a contradiction, since the oldest MacBook that you could possibly run Yosemite in is the:

How to install OS X Yosemite on your Mac - Apple Support

"MacBook (Late 2008 Aluminum, or Early 2009 or newer)"

We've been talking about this very problem:

Can I reasonably keep this laptop going?

Need to replace HD on Macbook early 09