i LOVE northern brewer. It's woody and earthy, very different from most hops. It's great in bittering, flavor, aroma, whatever...i use it throughout one of my trademark beers, my Vienna SMaSH:
10 lbs Vienna Malt
1 oz Northern Brewer hops (8% AA) @ 60 minutes
1 oz Northern Brewer hops (8% AA) @ 20 minutes
1 oz Northern Brewer hops (8% AA) @ 5 minutes
Best stuff ever.
Fuggle is my favorite hop for english style beers and stouts. I'll use it for bittering or flavor and often finish with East Kent Golding, which smooths and softens the beer while accentuating the chocolate notes in stouts. These hops are very earthy.
German "noble" hops are often used for hefeweizens, lagers, etc. They are usually low in alpha acid and big on aroma. They generally have a pleasant spiciness. Despite their low alpha acid, they can still be used as a bittering and it is a common practice of mine to use only a small amount in bittering my hefeweizens and dunkelweizens...just enough to balance the malt and provide a slight spiciness. Examples are Tettnanger, Hallertau, Saaz, etc.
I've very often used saaz at the end of all types of beers to soften them up.
Belgian beers can really use anything, as they are extremely varied styles, but Styrian Goldings (a fuggle variety from slovenia) is a common hop that is extremely soft and subtle...perfect for beers where you want some flavor but the maltiness is dominant.
I'm not a huge fan of american citrusy hops, but amarillo is probably my favorite flavor of the bunch. The others include the big C's like centennial, cascade, etc.
American hops also have lots of different varieties that are comparable to english and other strains. Willamette, for instance, is a fuggle variety and Liberty is a hallertau strain.
Definitely do some reading yourself, but brewing and tasting is what really will help you to understand ingredients. Some of the old "replacement" suggestions that are floating around the internet are not always the best source of information...but they will offer a guide and help you get started.
One thing that I did some experimenting with not long ago was blending hops. You can get really subtle flavors and accomplish incredible things with blending. My friend is a master at it...for instance, blending something with styrian golding to soften the flavor...or mixing earthy and citrusy characteristics.
It's a load of fun as long as you don't overdo it. I personally am not a hophead, so I enjoy the subtleties and balance that hops provide.
I would also advise you to pay attention to your alpha acid content and get some brewing software (promash/beersmith) to better work out your bitterness calculations. you can get many different hallertau, for example, ranging from 3% and 6% alpha acid content. You use those seperate levels in the same recipe and you will get two completely different beers.
lastly, you've already made a huge step coming on HBT. Never be afraid to ask a question here...besides experience, this is the best tool out there for building knowledge.
Hope this helps!
EDIT: One more note...using high alpha acid hops is common for bittering. It's cheap and effective and not meant to impart much flavor. I have a bad habit of using lower AA hops for bittering just because i love the subtle flavors they add, but I'm working towards breaking this