Learn Farsi: دانشجو باید وسایل را برای فردا آماده کند. - The student must prepare the things for tomorrow.

The word باید means must and it always forces the main verb into the present subjunctive form. Because of this, the normal present tense می‌کند drops its میـ prefix and becomes کند.

I learned that subjunctive verbs start with بـ (be-). Why isn't it آماده بکند?

For simple verbs, you usually do add the بـ prefix. However, آماده کردن (to prepare) is a compound verb made of two parts. With compound verbs using کردن, Farsi speakers usually drop the بـ prefix in the subjunctive. While آماده بکند is technically correct, آماده کند sounds much more natural and is far more common.

Why is the word را sitting right in the middle of the sentence?

The word را points out the specific direct object. Because the sentence is about preparing the things (a specific set of things) and not just general things, Farsi requires را immediately after the noun وسایل (things).

Does باید change its form depending on who is doing the action?

No, and that is what makes it an impersonal modal verb! باید stays exactly the same whether it is I must, you must, or the student must. The person doing the action is only shown by the ending of the final verb, which here is کند for the third-person singular student.