"Looking up" some information means searching for somewhere like:
- on the Internet
- in a book
- in some files
Most of the time, you use the phrase "look ___ up" when you were successful at finding the information that you were looking for. So you'd say something like this if you weren't successful:
I tried looking it up online but I couldn't find anything useful.
Like a lot of phrasal verbs, "look up" can be separated by words like "it", "them", "us":
Don't believe me? Look it up.
But longer words come after "up":
I looked up how far away the closest star is, and I was amazed at how far it was.