This is a great question. As you learn to create calculated fields in Tableau, there are a couple of things that may help:
- Understand the 3 main types of calculations: Row-Level, Aggregate, and Table Calculations. (There's also Level of Detail, but circle back after you have a good handle on Row-Level and Aggregate) If anyone knows a good resource that really explains these 3 types clearly, please share! I do give a treatment of these in this book: https://www.amazon.com/Learning-Tableau-10-Joshua-Milligan/dp/178646635X/
- Bring what you know. Do you know how to create calculations in Excel? Great! There are some similarities and differences. Are you comfortable with SQL functions? Outstanding! The syntax is very similar (even identical in many cases) and you'll have some idea of set based logic along with row level functions and aggregations. Have you any experience with Java, C#, C++, Visual Basic, etc...? Good! You'll have understanding of data types and logic statements. Have no programming background? Great! You won't have to unlearn anything because Tableau's paradigm is slightly different than everything listed above.
- Practice! As you start growing in your understanding of calculations, find examples online and work through them. Post specific questions on this forum and you'll get specific answers -- you have a whole community eager to help you. And then, try answering questions on this forum. Admittedly, many of the calculation questions are hard, even for the "experts", but if you try and fail and then see someone else answer, you'll learn more than ever (that's how I learned the most).
I think you'll definitely be able to grow in your understanding and confidence. Not only will it be sustainable, but you'll soon find yourself the "expert".
Hope that helps!