Almost forgot to post this one! Before you start making your game, it is very recommended to write down your ideas in a piece of paper, or type them down on MS word. For example, you want the multiplayer battle feature in your game where the player must choose the enemy first. You'll need a name for the feature, because 'Multiplayer Battle' on the menu just sounded pretty dull. So, you might want to name it 'Battlefield', or 'Asteron Arena' (for outer space themed games), or stuffs you can create on your own. Then the facilities headquarter, you may name it 'Kingdom' (like in Dracoterra), or 'Camps' (like in Archaeozoic Area), or 'Headquarters', or 'Ship Stations' (like in Operation H.E.L.D), or such stuff.
Now, while your ideas are popping out like popcorns, write them down quickly! Now that you have the basics you'll need, we can start from the beginning (check Step 1: Preparing the CSS Files to start making the layout for your game).