If you're using a kit, there are things you can add to raise the SG (and therefore the ABV) of your beer. These additions will usually change the taste of your beer to some degree - sometimes better, sometimes worse. Pretty much anything with fermentable sugar added to the recipe before fermentation will work - cane sugar, corn sugar, brown sugar, honey, molasses, fruit, etc. Most home brewing stores will also sell "booster" or brew enhancer that will raise the alchohol as well. Be careful, though - if you use too much of these your beer can get thin and cidery tasting.