Thy shall test thy level
It is very important (vital) that you test your level at least 5 times before releasing it to the world, make-sure to do things that aren’t what your supposed to do. To make your game even more bug proof, make someone else test it because they might do stuff that you'd never think of.
Thy shall use variables
Make-sure that you use variables often as they allow you to create more complex levels and save you countless hours of work. Using variables saves you time because, say you've asked the player for a username, you'd save this in a variable and use the variable to update leader boards etc... Instead of repetitively asking the player for their username - they also allow you to change important values quickly instead of searching through lines and lines of code.
Thou shall look at other games
Before making your game, have a look at what others have done. This will allow you to compare and make your game up to the standard expected.
Thou shall use grammar and spelling correctly
There is no point in making a great game and have spelling mistakes, this will effect what people think of your game negatively. This is also the same for grammar. Make sure that grammar and spelling are correct.
Thy controls are sensible
When making your game, consider the controls. You don't want somebody pressing control and shift for everything because these controls are difficult to access and (for Windows users) will make sticky keys appear. This will make the player get frustrated very quickly and vote poorly for your game.
Thou shall use sounds
Sound are very important when making a game, with-out sound a game is just boring and not very fun. Sounds make all the difference. You can get sounds from lots of different websites, I use Free sounds because I can use their sound effects as-long as I don't sell the game I am making.
Thou shall check-out tutorials
Before making your game, check-out some of the tutorials. They may teach you things that would of took you hours to figure out.
Thee shall create a clear UI
So, you've got a great idea for a user interface (or HUD). Make sure that the player can actually use it to check important information, like; health, score and ammo. Don't use hard-to-read colours and/or fonts.
I hope that you have found these commandments very useful, thank-you.