You can practice swich stance, that helps you a lot when you start to land them in backwards, but otherwise i'd reccomend you just to go for it. It took me quite a lot of time to learn the Vulcan so don't give up!
The fin size really depends on the boards and sails you are using and your weight, so it'd be good to know that to tell you what size to get.
Generally a freestyle fin should be about 18-22 cm long. For me the important thing is that the shape of the fin is right (it's not too good when you are using a wave fin...).