How to do a proper A/B landing page test?

I’m wondering how do I properly perform a split test (so called A/B test) on a landing page?
How many visitors do I need to have to keep the results statistically correct?
What goals can be set during the test? Should it be conversion boost or something else like less bounce % and more on-page time spend?
What changes should I do? Which changes can be taken “significant” and which are not?

I know these questions may seem silly for a pro, but to me it is very difficult to get answers. The Internet has many blogs posts on the topic so it is easy to get lost. And I was never good at finding information :fearful: