You are already off to a great start. When you get stuck on a task, always feel free to reach out on the forums for help. Employees, like myself, as well as a large community of users/customers regularly browse the forums to help out where we can. Keep in mind that your requests will always be answered quicker if you provide packaged workbooks, descriptions of the issue and desired solution, and screenshots if applicable.
As far as training goes, I would start with some of the introductory videos here: Training & Tutorials | Tableau Software
These videos will give you a good overview of how to get started with the product. The advanced videos will help you focus on learning specific features and analytic methods of Tableau.
Overall, there is no required knowledge set for Tableau users to be successful; the required knowledge is completely dependent on how you have to use the product. Basically, do not worry too much right now. Try your best to learn the product and reach out when you need help. It is as simple as that.
I hope this helps,
I agree with Dan, particularly his last paragraph. You can do amazing things in Tableau without knowing a bit of SQL, and at the same time there are some things that are near-impossible to do in Tableau without reshaping your data and maybe using SQL to do so. Where you end up depends on what kind of data you are working with and what kinds of visualizations you are trying to create.
It seems to me that one useful categorization is "users who have (mostly) clean data" vs. "users who don't". If you're connecting to a data warehouse or other source of relatively clean, organized data, then users can go far and fast with Tableau. If your data is messy or not in an analytic-ready state, then Tableau can make that process faster but there's still some amount (a little to a lot) of work involved to clean up your data, and that's one place where having a skill set in data modeling, Excel functions and/or SQL can tremendously speed up the process.