I have a few GB free on my hard drive, what can I do to increase this?

I have a few GB free on my hard drive, what can I do to increase this?
I currently have less than 20GB left on my hard drive and I have deleted all I can, I use a time machine back up disk and don't want to buy am external hard drive. If there a way I can buy more memory to increase the speed my macbook runs at, at the moment if I open numerous applications it significantly slows the speed of my macbook down which didn't use to happen.