Check out the #makeovermonday hashtag on twitter. Andy Kriebel and Andy Cotgreave run an excellent opportunity to learn more about Tableau. You can read more and get the data here: Makeover Monday Challenge
Also check out the tips and tricks sections on Andy's portfolio. You will learn a ton there. He's got a bunch of videos too. There are many other places to learn and blogs to read, but if you had to pick just one, start with Andy's.
You can check www.tableaulearners.com and there are many similar website have good tutorial.
You can look for the data visualization course on Coursera website. It would charge for the certification. You can ignore certification part and go for the course content. They have really nice study material and practical to get hands on experience on Tableau.