how do you save your game?

Throughout the game, you'll see various green circles on the ground. There are two types of save points, but all you have to know is that they're green, and they're scattered throughout the game.
For example, if you're doing the opening tutorial part of the game, where you're on the platforms with the mosaic-like designs of Disney movies, a green circle will eventually show up. Just step onto it, then scroll down your Command Menu to select the 'Save' option.
Another example is inside of the shack on Destiny Islands. When you get to the point in the game where you're controlling yourself on the islands, go into the wooden shack, and there's a save in there. Again, just select 'Save' on your Command Menu.
If you have any other questions, just ask them here, and I'll try my best to answer them.