How to approach the implementation of auto-tests in the project?

Question

Automation

Good day. I work in a bank (all the accompanying rigmarole in the form of increased security measures, etc. is attached) in a team that creates a new website for the company. The project is a little more than half a year (if we take the time from the launch of the site). A lot of things happened to this project before I, an ordinary chiropractor, got here. Until recently, there were no individual testers on the site at all, they took people from other teams for a couple of days in order to quickly drive away the functionality so as not to fail on the release. After digging into the documentation and supplementing it a little, I came to the conclusion that for regression it is necessary to run about 400 scenarios, with the supply of fixes / functionality every few days, it becomes somewhat stressful. I suggested to the authorities to think a little about how to implement auto tests. Well, as expected, I was told well done here and do it,
Actually a question. Please advise me what might be best suited from the existing tools, techniques on this topic. Everything will be from 0, so there is where to roam (the main thing is that it would be free :) )
I am a beginner in this area, they gave me a lot of time for this. I also really want to develop and it all sounds very interesting.
I will be grateful for advice. Thanks