chiageraldine“Should I say "Fix this room for the guests." or "Fix up this room for the guests"?”
You fix something that is broken; you fix up something that is not tidy or orderly. So I would say
Fix up this room for the guests. Actually, I would say
Fix this room up for the guests. 
Concerning your website (I assume that's what you meant), if the site is not working at all, you need to fix it (or have someone else fix it); if it works but needs to be tidied up to look nicer, you need to fix it up (or have someone else fix it up).
There are other meanings for these verbs, so it would be best if you consulted a dictionary.
CJ