Creating Bubbles from Scratch

We will start off with a blank canvas, but choose a nice color, image or gradient to fill it with, like so:

Now we will create a selection of the shape of a bubble with the elliptical marquee tool. It doesn't have to be a perfect circle - just the shape of a bubble:

Now create a new layer and fill the selection in with any color; I chose White:

Reduce the Fill of this layer to zero. You will find this option on the Layers panel, below the Opacity option:

This will make the Layer's contents invisible, but what we are going to do is apply layer styles, which will still show through. Open up the Layer Styles either by right-clicking the layer and choosing Blending Options, or double-clicking the layer in the Layers panel. You will want to play around with these settings to get the best outcome, but add a Drop Shadow (Opacity around 10%, Distance 1, Size 5), a Inner Shadow (Opacity around 10%, Distance 3, Size 5), and and Inner Glow (Color: White, Opacity 35%, Size 15px). You should have something like this:

Next, choose a soft brush around 9px or relative to the size of your bubble. Create a new layer and paint an arc at the top left of the bubble. Then reduce the opacity of the layer:

Try it yourself and see what you can come up with.