Depends on the style of beer and what characteristics you like within that style.
Brew Your Own has a wonderful little guide to hops that is sorted by style of beer. I use this as a basis for hop combinations and the knowledge of what works best with what style but it is a great place to start...
http://byo.com/resources/hops
Homebrewing really is about experimentation once you know the basics though. All your hops will stem from the style you choose though. English Pale Ales us UK Goldings and Fuggles most likely with a little variety here and there. American Pale Ales and IPAs use hops like Amarillo, Cascade, Centennial, Chinook and other floral, citrus type hops. Stouts and European Ales use noble hops with lower Alpha Acids that typically won't overpower the flabor of the beer.
Do a little reading about what style you are brewing and hit up the BYO hop guide and you can easily get a handle for what kind of hop combinations work well together. Then from there it is experimentation and experience that will help you hone in on the flavors and aromas you enjoy and what best suits your beer.
The other side, once you have an idea of what type of hops you want to use is the timing of the additions. How late or early in the boil you want to add them, whether you want to dry hop? It's all up to the recipe and you.
Bittering hops: Typically higher Alpha Acids (11%+), although not always (noble hops in stouts and european ales sometimes top out at 5-6%), added early in the boil between the start and about 30 minutes. These will give your beer a bit of a bite depending on how much and how often you add them.
Flavoring hops: These are added between 30 minutes and flame-out. They will typically fall within the mid Alpha Acids range between maybe 5% and 13%. Since these hops are boiled for less time, their bittering agents aren't broken down entirely so your beer gains most of the hop flavor with out so much of the bitterness.
Aroma hops: These are typically lower Alpha Acids (3-8%, depends on style). They are added right at the end of the boil or are dry-hopped in secondary fermentation and left in for anywhere from 7 days to a few weeks depending on the quantity. Too much for too long will yield vegetal flavors. India Pale Ales pride themselves on their aroma which they receive from dry-hopping.
So know your style and read up on your hops. There is plenty of information on this site and others if you google around. Typically if you search for the style of beer you will find something about the type of hops that best fit it. There are plenty of more knowledgeable people on this forum but this is my two cents.
Cheers!
