what you want to do is not really meant for first time users. It requires that you use a selection tool to select what you want to keep, then move or copy that selection to a new layer. You then hide the old image by clicking on the eye next to the image in the layers panel. You now create a new layer then move that layer below the layer your keeping. In that new layer you fill it with any color you want.
The reason why this is meant for someone that has used photohop for awhile, has to do with how well you can make a selection. There are various techniques that can be used to refine that selection so that only what you want is kept and stray pixels that do not belong are not selected.
{For practice only}
You may want to practice for a bit, create a new document, fill the document with a gradient, then put a shape in the middle of the document. Now rasterize that document.
Now you can practice selecting that shape. For more advanced practicing, use a custom shape the more curves in the shape, the more advanced your practice will be.
For more advanced practice, apply a slight blur to the document, this will feather the shape into the gradient, making more difficult to get a good selection.
This will help you try out all the selection tools and mask options like refine mask. Just play around, watch as many learning videos that you can.
(free)
http://creativesuitepodcast.com/
(pay)
