When beginning to write a theme or accompaniment for any kind of visual, the most important thing to consider is how the music will complement the end result. Think of a main music melody, a piece that will serve as a repeating theme in the work. This is easily accomplished when the tone and theme of the game has been determined.
A dark game should have a dark theme, a children's game should sound bubbly and fast, a puzzle game introspective and complex, a platformer energetic and straightforward. There are, of course, exceptions and innovation only occurs when risks are taken, but generally the thing to keep in mind at this early stage of progress is to make a soundtrack compatible with the end result.
With the theme and tone of end result in mind, develop the melody. It should have at least one to two minutes of original content with a pleasant, driving turn-around at the end so repetition of the main theme will not sound jarring as it repeats.
The melody ought to be catchy without being annoying, and musically complex enough to interest those with a background in theory. Perhaps the game that best exemplifies this kind of musical writing is Squaresoft's Final Fantasy series, which has inspired fans to learn character and game themes on piano. There is even a canon of Final Fantasy themes available in sheet music form for piano or strings.
Now that you have a basic theme for the project, use similar arrangements or instrumentations to create the other tracks needed for the game. It's a good idea to have a dramatic theme, a menacing theme, a theme for triumph and defeat, and whatever else this work calls for. All tracks should be loop-able and maintain some cohesive quality when played alongside the other pieces.