First get training in tableau and start your job.
you can study tableau online videos available in tableau website.
1) Start with Tableau Free Training
2) Read out Tableau Help docs and KB's
3) Follow forum, work on problems, check the discussions
4) Read various blogs
All the Best for the Start!!
Even i had started my Tableau journey 1 year back and now i am a Tableau Desktop certified .
All you have to do is go through the training videos Tableau Training: View Training Courses (Trust me these are the best) ,be active in discussion forums and do lots of hands on(scenario you can get from forums only ).