A good starting point would be to look into the hop bill of beers that you like...
It's a really subjective thing, what you consider to be a great combination may be ordinary to another.
Classic combo's include all the American "C" (including Amarillo) Hops. For English hops Fuggles and Goldings...
Find a hop(s) whose description you think you will like than google for substitutions for that hop(s). The substitutes will generally compliment.
Designing Great Beers (Ray Daniels) is a great source for info on what hops have been used in a variety of styles in both commercial and home brewed beers