1. Training & Tutorials on Tableau's website
Select 'On Demand', you have to create an account but it's free and you don't need to be a Tableau customer. The tutorials are selected into few groups Intro, Advanced, Chart Types, Server. It's the best place to start learning.
Check 'Live Online' out too.
Tableau's Knowledge Base – hundreds of tutorials are divided by topic.
Each short tutorial gives answer to a problem, it's good to check the titles and do tutorials which can be useful in your work. The list look like a never ending story but no reason to be in stress as most of things you will learn doing 'On Demand' tutorials.