Learn Arabic (Modern Standard): ما أخذت المال أبدا.

You are right that ما often means 'what' in questions. However, when you put ما right before a past tense verb like أخذت (I took), it acts as a negation word meaning 'did not'. So, ما أخذت simply means 'I did not take'.

Where is the word for 'I' (أنا) in this sentence?

In Arabic, you often don't need to use the pronoun أنا (I) because the verb already tells you who is doing the action. The letter ـت at the end of أخذت specifically means 'I did it'. The pronoun is built right into the verb!

I learned that أبدا means 'never'. Why is it translated as 'at all' here?

أبدا is a word used to strongly emphasize a negative statement. When you use it with a past tense negative sentence like this one, it translates best as 'at all' or 'ever' (as in 'I didn't take it at all'). If you use it with a future negative, it usually translates to 'never'.

Why does the word for money, المال, start with 'al'?

The prefix الـ (al-) is the Arabic word for 'the'. The base word for 'money' is مال (maal). Adding الـ to the beginning makes it المال (the money), showing that the speaker is talking about a specific sum of money.