How would I go about doing this? I've done some Google searches but i'm finding it all a little hard to understand. Can someone help me out?

 

The reason I ask is because many of the programs I frequently use take a long time to start up, whereas 6 or 8 months ago the whole process was a lot faster.

You could just do a complete reinstall if you have a good way of backing up your files. I have a Time Machine, which allows me to really easily back everything up. I did a complete wipe of my HDD and then reinstalled the OS and my Mac has been running significantly faster since then. But as I said, this method is really only easy if you have a Time Machine or another really reliable way of backing up your stuff... and it does take a long time to restore your files so it's definitely something you'd want running over night.

I don't have external storage available.

I know Macs do an automatic defrag on start up.. but I think it's only for small files. Maybe try out Cocktail? I haven't tried it but I've heard it's good and it has a free demo.