I'm looking for some carbon fiber DH or XC wheels and don't know where to start, as I'm new to this. Could you give me some help on where to get started?
For XC you can find a myriad of wheels that are now carbon.
For DH, you can find some examples but they are big money.
They're typically more expensive than their aluminium counter parts, but a little lighter. Rotational weight is typically reduced, which will make the bike turn more quickly and feel "snappier".
Since you're new at mountain biking, I wouldn't recommend getting top spec stuff right away. You're going to have "oops" (e.g., wrong trail, endo, rip a dérailleur off, etc) and it would put a serious dent in your pocket book to replace carbon wheels. You can get two decent bikes for the cost of some carbon hoops.
Carbon typically isn't as forgiving in crashes because of the nature of the element. It's more prone to damage from impact than aluminium or steel counterparts, and the failures can be much more difficult to detect. Just my 2 cents.