Did you take many statistics classes during undergrad? If not I’d start there. But if you feel like you have a good grasp of basic statistics,
I suggest learning Rstudio, Python, and SQL.
* This book is excellent for learning R.
This book is a really good starting point for Python.
Popular alternatives to
Hands-On Programming with R...
An Introduction to Statistical Learning: with Applications in R (Springer Texts in Statistics)