You just need a little bit to build on. You might have a hard time remembering entire dreams when you wake up at first. When you wake up in the middle of the night, just write down anything whatsoever you remember seeing/feeling/touching/smelling/etc. Try not to move when you wake up for the first few mins. Sometimes you'll get bits and pieces of you last dream that will come a few minutes later. Get up and go to the bathroom/grab a quick drink and try thinking about what you were last dreaming about. For some people, albeit less, it will help if you get up for a minute and try hard to remember. Also, if it's on your mind, some people will remember dreams over the course of the day if you try to.

A last alternative would be to take a few minor supplements to help a little bit. Most B vitamins will aid you. (B5, B6, B12)