There are several options.
Transfer much of the stored data like movies and music to an external drive and the erase it from the MacBook.
Then upgrade MacOS X to 10.6, 10.7, or 10.8. depending on which 2008 model ( plastic or aluminum case ).
Add RAM to the maximum of 4Gb
Get a larger solid state hard drive.
Or consider buying a new one. A new Mac would be light years ahead in performance compared to anything you could do to improve the 2008 MacBook.