Question about taxes Data Analyst Portugal

I just got accepted as a Data Analyst Portugal and they gave me the green light to rate but I didn't yet initiate activity as self employed / independent contractor I didn't start rating yet afraid to fuck something up in terms of having done tasks prior to activity starting regarding filing all my earnings I wonder is it a concern?

Im going to take care of that tomorrow in the morning I would have done it sooner but I wasn't sure if i was gona be accepted or not they just simply send an email saying im in and I am allowed to rate.

Also what kind of sector should I choose when opening activity as self employed? what kind of sector is Telus Digital Labeled under?

And won't I need the NIF / VAT of the company when emitting the green receipts since im supplying services to a company?

Sorry about all of this but I have never done it before and I am kind of in the dark and telus doesn't explain very well what kind of measures I should take besides saying I should run as independent contractor they could give more information but im gona try and ask all the questions tomorrow when starting activity.