Firstly you'll need to download and install Roblox Studio, a free piece of software that is used to create all Roblox games. Hit the "Start Creating" button on the homepage, and then the "Download ...

According to Roblox Studio, all users are free to create as many experiences as they wish, but each account is only allowed to have 200 publicly playable experiences available at any given moment.

Development is outside of the scope of this article, so for that you should visit Roblox 101: How To Make Your First Game, which details what you need to know and where to find that information.