First, ask yourself what you are trying to accomplish. Your design should be different if you are providing knowledge as compared to persuading the player. Then, play a few games that are successfully accomplishing what you are attempting to do. After you've played those games make a list of what makes some of those games more compelling to you than the others. By that point you will hopefully have a better idea about the type of game you are wanting to create as well as some ideas about the design strategies you want to employ in order to make that happen. (If you have the time it would also be good to enter some "game jam" competitions. In addition to experience, you should also get some valuable feedback.)