There are a handful of good resources available when you are looking for help.
Probably the best place to look when you don't know how a built in IDL routine works is to look is at the Alphabetical List of IDL Routines.
IDL Programming Tips, a frequently updated site maintained by David Fanning, is just a really interesting place to spend some time reading. Not only will it have in depth explanations to some problems you are likely to encounter, such as floating underflow error, it has answers to questions you probably will not even think to ask. In addition, many answers give insight in to what is actually going on behind the code. I may even assign reading here from time to time.
Another option is to come in to 705 Campbell. The bookshelf by the door has a number of books on IDL and Unix. There are generally students in the lab working who can be persuaded to help too.