I want to start and register a small promotional advertising company.

This is a very important step before doing anything else. I would like to suggest you make a plane first like:
What are your objectives?
what is your audience?
which advertising do you need to be a focus on?
What is your budget?
Your rules and regulations?
Your target that you want to achieve in the first year?

All of them need to be sorted and then make a plan. This will help you a lot!