It depends a little, but the simplest case is when I want to make a beer that is in a clearly established style. In that case, I'll look at the BJCP style guidelines and a few model recipes. The model recipes are usually from this site and/or from one of my trusted brewing books. If I don't know the style well or haven't brewed it much, I stick pretty closely to the BJCP ranges for IBUs, ingredients, etc. the first time.
But the key question is why you're making your own recipe. You need to have something in mind when you start--are you trying to feature a certain hop, or grain, or spice? or exploring the space between two styles? or seeing how dark a beer you can make without getting something disgusting? To help get an answer to those kinds of questions--and really, to figure out which questions I want to ask--I usually taste a few commercial examples of the beer type I'm going for. Then I can get a sense of what a well-made IPA, or brown, or saison is like.
Of course, sometimes I don't want to make something within a particular style; sometimes I want to make something that tastes a certain way, or features a certain oddball grain. For example, when I made a beer that featured guava, I didn't have a style in mind; instead, I had an ingredient in mind. So I had to decide what elements (grain, hops, yeast, spice) would complement that fixed element.
So, I dunno. It's a hard question to answer because it's so broad. Maybe it's easiest if we start with that first question: what kind of beer do you want to make? If you don't know the answer to that question, you should probably think about what beers you like--that's usually a good starting point.
