You can start with free video tutorials on tableau website itself. Below is the link:
Do the practice along with the video and make sure you clear all the concepts in a video and then move forward. You can post questions here on tableau community in case any concept is not clear from video.
Spend at least 1 hour daily. You should be good in 1 month time. All the best!
If you want to get good training then workout community questions daily.