For general reference, this is a good guide to centring things:
However, as Erik says, if you are using Bootstrap, you will need to check the documentation and work within the framework.
I would question whether Bootstrap is necessary or appropriate in this instance. It is not like a site builder, which makes it easier to design something with little or no coding knowledge. Bootstrap requires a good understanding of both CSS and the rules of the framework. I would suggest you forget about it until you are much more familiar with CSS, especially for a small site.
And why are you still using <br> tags for spacing, when you’ve asked about it and been told that is an incorrect use? Use CSS, and if you don’t know how to do that, then ask for guidance.