In designing a video game there are a few things you must know. You cannot just wake up one morning and say, "Yeah! I want to make a game". Developing a game is much more like building a house. To build a house, you need a plan and the necessary materials ready.
Before you start, you should ask yourself, "How good is my geometry?" "Do I have a fair understanding of physics?" "Can I code enough?" If yes, yes, and yes, you are good to go! And if no, you should take your time to study and understand the basics of displacement (http://wikipedia.org/wiki/displacement), speed (en.wikipdia.org/wiki/velocity), and the application of coordinate geometry (http://www.math.com/school/subject3/lessons/S3U1L2GL.html).
Since we are considering 2D games, we should be more concerned about the x and y axes of our game objects; the z coordinate is out of the question because we we will only need to refer to that in 3D.