1. Programing
Python and R are both good for data science. If you want to become core data miner then prefer using R and if you want to go for core data science/Al {ML, DL, NLP, RL, CV} then prefer using python but still this is a debatable topic.
2. Theory
Core concepts of Statistics, Probability, Linear Algebra for machine learning, deep learning, natural language processing.
3. Data Visualization
Tableau and PowerBI are commonly used in industries, to make state of the art visualizations and they are easy to use. Whereas matplotlib/seaborn/plotly/bookeh are useful when we want to have quick look at the data.
4. Big Data
For big data, you can use AWS/GCP/AZURE for storage and Spark/Pig/Python for processing. Here you will need to pull lots of data from SQL and NoSQL databases.