This website (al-islam.org) has an in-depth library, and you can find materials which are both introductory and advanced. So, I recommend starting here!

You could also look online for taught classes, since sometimes it is useful to have a human teacher. 

Beyond that, there is also ample material on YouTube - as a hint, if you want to find a discussion of a subject, search for Muharram majlis followed by your subject, and those are almost guaranteed to be presented by Twelver Shi'a. 

Best wishes!