First of all, pick your two colours you want your bubble too be. I chose a dark and light blue. Then go to Filter> Render> Lens flare, and use the default settings(I forgot to do this, this will make the bubble look shiny). Then go to Filter> Render> Clouds.
Step 2: Ooh, Polar Coordinates!
Now go to filter> Distort> Polar Coordinates. Then Edit> Tranform> Flip vertical.
Step 3: Finishing the Bubble
Now you have your bubble, but it is a bit hard to see. Too make it more visible, go to your elliptical marquee tool (top left tool) and drag from the top left corner down to the bottom right corner. Now hit ctrl+Shift+I (for a mac I think it's command+shift+I), then hit delete.
Step 4: Tada!!!
Your bubble is done!
(PS, I can't make image notes, do I need Mozilla Firefox, or can I use Internet Explorer?)