The Mini should have a pretty straight forward "add/remove software" utility where you can see programs that are installed and decide what to keep and what to uninstall. I'm not familiar with Dell's Ubuntu to know where to tell you to look for that utility, but in normal Ubuntu it is in the main menu below all the program groups (like Accessories, Internet, etc.)
If you're not sure what a program is or does, google the name to find out.
Stuff you're not sure about, leave it for now. Get rid of what you know can go and then go from there.
And for the record, what size SSD
do you have?