This looks like a pretty good start. My only thought is that this world with so little land is going to have a very wet climate. If you're ok with that, great, but if you wanted more earth like conditions you may need more land masses.

If you want to fill out your map a bit more, I'd recommend picking a tutorial and going with it, eventually, you'll start to pick out elements you like and developing your own style. The quickstart guide has links to several tutorials, and you can choose the one that looks closest to how you want your map to look. Keep working with it and tweaking it until you have it how you want it, the best way to learn is by doing.