I haven't heard of Udemy until now. I looked around on their website, and I agree with @Fairytale Studios. It looks like they'd be a good place to start, but not as a substitute for practicing. If taking a class is what gets you to practice, then go for it!
Maybe it is just me, but I have had a pretty negative experience with Udemy in the past. I know there are many instructors there, so it was that course I took that I didn't like. Your experience might be different.