I think you should do all the things that Maydup says first. They are very easy, although they may take some time. If you haven't done this "clean-up" exercise for a while, or even ever, you should see a good improvement in speed. But be prepared to be patient....a defrag can take hours !
But you haven't said exactly what you mean by "my computer is very slow"
Slow doing what ? Downloading a file for instance ?
What download speed are you getting ?
Are you on a fast broadband service, like BT's Infinity ?
If you don't know, do a speed test first ....try this one for size :::
http://www.broadbandspeedchecker.co.uk/