I started with books, the first Planet Cake one to be specific. It gives you really great info on the basics like how to ganache your cakes perfectly and how to prepare and use fondant etc. I know ganache completely differently but it was awesome for getting started. Debbie Brown's 50 Easy Party Cakes is also good for when you're starting. I now have a massive cake decorating book collection, some get more of a workout than others, and there isn't one that I haven't learned something from.
There's pretty much a tutorial for everything online, whether it's a blog post or youtube video. I haven't got much out of cake forums but I am a member of several groups online. Even though I've been doing it for a few years now and I run my own business I have just signed up to Learn Cake Decorating Online
. It's an Australian site and is $9.95 a month. I did a four tier ombre ruffled wedding cake last week and I learned the technique from their ruffle video. Another site I'm signed up to is My Cake School
. They are based in America and their fee is yearly but I would happily pay double because their tutorials are so worth it! Cake decorating classes on Craftsy
are also awesome and if you sign up for their newsletter they regularly send out emails with links to discounted classes.
So I'd start with a few books and go from there.