Please check out Roblox Studio [1], the free Win/Mac IDE for creating and publishing games on the platform.
Also look at the developer site [2], where you can find documentation, tutorials, and an awesome, active community in the developer forum [3].
[2] https://developer.roblox.com
[3] https://devforum.roblox.com
(disclaimer: I'm an engineer on the Roblox Studio team)