Q: How do I free up memory on my Macbook so I can upgrade to Mavericks?

You have to buy more RAM and install it.

Plenty of online instructions that are easy to follow.

FDC --

You need at least 4 GB to install Mav.  It is very reasonable in cost now.  I get mine at OWC.   Get the most you can afford, and check out the video instructions for installing RAM in your particular Mac.  You will be very pleased at the difference in speed.

2008 macbook? Leave it on Snow Leopard in my view.

Buy what you can afford, but you need to buy two identical boards, eg 2+2 ,

Here are the hardware requirements for Mavericks:

So, if yours is a late 2008 Aluminum, you can do it, but you have an older, slower processor, etc, so I'd stick with Snow leopard (however, increasing your RAM would probably be of benefit).