a) what you want to say about making believe b) what will look best. Sometimes what looks 'good' in your head looks crap on paper so a few sketches wouldn't go awry c) how obvious/literal you want to be. The quite obvious option is stars and wishes and magic and children dressing up in costume, then you can start to drift away into something more like film and TV make believe, 'grown up' acting. Or what about making believe in the sense of hiding something. Making believe everything is ok when we're falling apart inside. Or... well ideas just spill one over the other. First pick an idea, THEN start to think about it visually.
However what you have to remember is whatever you do it cannot possibly be "wrong" and it might come out "shit" but that's something you have to learn as a creative, what ideas work for you and what don't. I could say I hate idea X but idea Z is awesome because in my head I could do that one, but for you maybe idea X is better and you could make that one better. You just have to pick one and go with it full steam ahead!