Learn Arabic (Modern Standard): ماذا تكتب؟

It is built right into the verb! The prefix تـ at the beginning of تكتب tells us the subject is "you" (masculine singular). Because the verb already shows who is doing the action, we don't need to add the separate pronoun for "you" (أنت).

Why do we use ماذا for "what" here instead of ما?

In Arabic, the word for "what" changes depending on what follows it. We use ماذا before verbs, like تكتب in this sentence. We use ما before nouns, like when asking "what is your name?".

Does تكتب mean "you write" or "you are writing"?

It can mean both! Modern Standard Arabic does not have a separate present continuous tense (like the English "are writing"). The single present tense verb تكتب covers both general habits ("you write") and actions happening right now ("you are writing").

How would this sentence change if I were asking a female?

You would need to change the verb conjugation. For a female "you" in the present tense, you keep the تـ prefix but add the suffix ـين to the end of the verb. تكتب becomes تكتبين, making the question ماذا تكتبين؟.