First things first ... decide what kind of track you want to make. Whether it be up-beat, pop, dark and gangsta, mellow, and so on. Set the tempo (speed) of the track to a moderate speed so you'll be able to construct the track with some ease.
If you don't have a melody in mind, what you should always do is start off with a drum line pattern. This consists of a drum pattern, a clap, hats, and/or any other percussion instruments. Based on the time signature (usually 4:4 count), it should be something catchy and repetitive. To get an idea of a drum line ... listen to any Hip Hop, Pop, or R&B song. Don't spend too much time on a drum line, most likely it will change by the time you're finished with the track. It's just a method of getting the juices flowing.
The next thing you want to do is come up with a melody. Now this is sometimes the hardest part of making a track. Melodies can be simple (consisting of 3-4 notes on an instrument) or they can be complex (consisting of chords, unison, notes). For instance, if you want to make an up-beat song, the melody can be simple with a few instruments; and a drum line, and easy to keep up with. If you want to make a pop track, you should come up a up-pity melody that's live and energetic. For a gangsta track, you want a rugged and dark melody. And so on.
Now that you have a drum line and a melody the next thing is construct the track. For instance a way to construct a track is to let the track build. Meaning, start off a few melodies, a drum line (or even no drum line) and as the song plays gradually insert the different patterns. Use variations. This gives it attitude.
When you "think" you're done (I say 'think' because a track never truly feels done). What you want to do is listen to the whole thing and as it plays, improv in your mind. This where the "ear" comes in. Think what instruments can be used where. What parts to take out. Basically what will make it sound better.
The last thing to do is...let people listen to it. Don't be offended if someone doesn't like it. This is an important step. It shows you what you can improve on, what works what doesn't and so on. But through all the criticisms, just remember to do what feels good to you. Before you know it you'll be constructing tracks in your head. The only thing is that, if you're away from your computer, you'll have to remember the track/melody long enough to construct it (lay it down). Feel the music!