Before you even think about writing a line of code you should make a design document that is detailed enough that you don't get lost, but not so detailed that there isn't any room for improvement. Some examples of design documents are: Al Lowe's(the creator of such wonderful games like Leisure Suit Larry and Torin's Passage) at http://www.allowe.com/gamedesign/index.htm and LusikkaMage's video on the topic at http://www.youtube.com/watch?v=IUDl52roY_w.
You may say to your self "I still don't see why I need a design document". Well think of it this way; if you don't have planning you will most likely build most of your game and then want to put in a really cool element, but you realize that you have to rewrite half your code to make it work. This will more than likely cause you to become less motivated.
So you want more proof? Well just recently I was working on a project with two of my buddies on a pong game. Sound simple right. WRONG! From the get go we made some concept art of what we thought the game should look like, but we didn't make a design doc on what we wanted. So when it came time to add in cool stuff like power-ups and menus we were at a loss and didn't know what to do. Also one of our guys was working on a graphics engine for the game which didn't get done in time to implement. So all these factors set us up for failure. We lost motivation to work and quit.
The moral of this story is that If you fail to plan, you plan to fail.
