Really good concept. I like the colours and the style you've made everything in. The composition is perhaps lacking. I think there should be swirls instead of bubbles since it would appeal more to the 'above-the-clouds- concept. Also you might wanna check out the rule of thirds, it helps with the placement of objects, will drag the audience to the right spots.
You are most welcome! Usually trial and error and most of the time from art books (I tend to read a lot about art). Experimenting is always good. Try incorporating new things to each design you make, it'll help your skill level and understanding of the canvas improve and widen.
I have only taken a couple of master classes and grabbed some more knowledge from DVDs when I wasn't in the mood for books. As for formal, nay. Never taken any classes. I honestly prefer DVDs, books and processes over classes, just my take haha