I'm going to take a test. " gonna" is mire for talking or texting

Hi Erika, "I'm gonna take a test" is the best answer if you want to use colloquial English. It's not like in Spanish where you use "hacer" to describe doing most types of activities.

Proper English is “to take an exam”. But you will hear people say “I’m doing my exam”, which is very informal, not really correct, but used anyway.

It depends on what you mean. I'm gonna take a test - you are the one tested (like when you take exams) I'm gonna do a test - you will test something (example: you want to test the speed of your internet connection.)

I prefer "take a test", but I think I've heard "do a test". Don't worry too much: English speakers will understand either way.