If you’re not an experienced linux sysadmin, or if you’re happy to just spend a little more on hosting, then using the hosting provider’s interface to resize to an instance with more RAM is going to be easiest.
If you want to try adding swap, perhaps see my post over here for some info and a link.
Edit: I see no indication that you’re short of disk space, so there’s no particular need to tidy up.
Edit: perhaps see also