Batz, read the bjcp style guidelines to get an idea of what types of hops are appropriate for the style you're brewing and read THE DRUNK ARAB's spreadsheet of hop types in this forum. Then grab a tool like Promash (or do the maths) to work out the quantity if hops needed to get the IBU's you're after. A little bit of experimentation is fine, but to brew good consistent brews, it's important to get the IBU:gravity ratio right for the style you're brewing... a little hard with kits, but you're a partial mash + extract brewer, aren't you?
Anyway, the basic rule is the longer in the boil, the more bitterness and less flavour the hop contributes. So to bitter beer, add "enough" IBUs of hops at the start of the boil and for flavour, about 15-10 mins before the end, and for aroma, add at flame-out or dry hop.