If it is the first time you do/ are doing/have done/have been doing it, you'll have to ask your teacher for help.
Suppose you will have to do one thing. Can we use both 'do' and 'doing' in this sentence? And how about if you are doing it?
The first time you do something you'll have to ...
If this is the first time you're doing something, you'll have to...
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.