Posted 02 June 2012 - 09:02 AM
You are posting in the wrong forum to get help on this, this forum is to discuss teaching Gamemaker (in schools etc.).
The best advice I can give is to forget your game idea for now seeing as you say you are new to Gamemaker, and instead go through some of the tutorials that come with the software, read the manual, read the excellent tutorials on here and then write some simple games such as breakout and other simple projects.
Once you have a few simple projects under your belt you will be able to answer most of your questions about where you start on your bigger game.
Its good to have a game goal in mind, but you need the underpinning skills to be able to work on it, without those you will be forever asking people where you go next, how to do this/that etc.