Hops impart bitterness, flavour and aroma to beer. Using some hops, and a little grain, will greatly improve the quality and flavour of your kit beers. Added hops will also help with head retention which is sometimes a problem with kit beers.
You can use either the packaged finishing hops 12 grams of hop pellets in an oversized tea-bag, or loose pellets. Loose pellets work out at much more cheaply, about 1/3 the price.
The first thing to do is to think about the beer you want to brew. Choose a hop variety appropriate to the style, and decide whether it is bitterness, flavour or aroma that you want from the extra hops because that will influence the method(s) you use.