You make an appointment in advance. They look at it and tell you what's up. If they have the parts, they can fix it on the spot. (but they will take it in the back where they have a static-free workstation). Fan replacements (if that is indeed the problem) are straightforward, but the parts are special Apple parts.
There is no need to touch any data, but service always begs the question, "Do you have backups?" If not, what if your drive died tomorrow?
If you have an external drive, Time Machine is good because it does the backup (when the drive is available) automatically at low priority in the background. It does not interfere with your work.