You need to do some basic reading on hops, alpha acids and how they impact brews so you know better how to control the hopping in your beers. Recipe calculators (to figure out the IBU of your beer) are very handy when you're trying to figure out how bitter a beer will be.
The short version is, the more alpha acid, the more bitterness a hop can contribute. Alpha acid percentages will be on your package of hops. You usually add the bittering hops at the beginning of the boil. By adding more or less of these, your beer will be more or less bitter. Or, by using hops with lower alpha acids, you can lower you bitterness. For example, a beer with one ounce of Columbus hops will be much more bitter than one with 4 ounces of Fuggle hops.
Hops added late in the boil contribute flavor and aroma rather than bitterness.
Hope that helps. Welcome to the forum!
