For Xamarin dev generally, I recommend
Creating Mobile Apps with Xamarin.Forms Book First Edition - Xamarin[
^] - it's free and very good.
But first, get C# sorted, which will take a while if you aren't familiar with the way .NET works - there is a lot to learn! Have a look locally for a course, and if you can't find one, get a book: Addison Wesley do some very good ones, as do Wrox, and Microsoft Press. Find one you like, and follow it from cover to cover, doing all the exercises.
Do not expect this to take an afternoon - it takes a long time, there is a lot there!
Do not bother with YouTube tutorials: there are probably some good ones there, but most of them are made by people with only slightly more knowledge of the subject that you, and are counter productive. It's very hard to find good ones in there with all the total rubbish...