There's a couple of ways to go about it, I'm sure. If you're not particular to any style it's just a matter of choosing your fermentables, hops, yeast and other flavoring ingredients (spices, wood chips, etc) and get to brewing. If you're interested in a particular style you can find a recipe for a beer and deviate away from it in certain ways to leave your personal signature on it, so to speak. Also, if you're interested in specific styles knowing what defines the style is, naturally, useful as well as knowing how ingredients add flavor to a beer. That means knowing the difference between the malts, hops, etc. Generally speaking you'll have more options with AG brewing compared to extract brewing.