What happens next?

How to make a gradient in photoshop
Create the shape of your gradient with the selection tools.
These are the tools with the dotted lines, like the square selection tool or the lasso, that let you set aside a small area of your image. If you don't select anything, your gradient will simply fill the entire layer you're working on.
You can make gradients any shape, as long as you can create the selection. If you're just testing things out now, make a small square and work with that.