There are lots of online resources to help you getting started with tableau, including many video tutorials here:
This website from the company I work for has a lot of useful links to blog posts etc.
And if you get stuck with a specific problem these forums are great for getting a solution from other Tableau users.
Hope that helps
Tableau opens up their past conference material for free online. There you can watch hundreds of presentations. Here is 2014's conference material:
Watching the video and following along with the tableau workbook or the dataset they provide has been an enormous help in mastering Tableau.