(Wordpress Basic Theme) To modify the post layout

Hi, how are you?

I opened a website by looking at the book "Wordpress Learning While Making" and am using the basic theme of Twenty-Seventeen

The site is being used in the form of a notepad, posting short sentences or words for personal use without a title.

What I want to modify is to 1. eliminate the date of each post being marked and 2. reduce the gap between each post and separate the sentences with a br line

I have no idea what file to use to modify the basic theme, so I'm asking you a question

I'm sorry that it's going to be complicated if you tell me this process like this. I would like to ask you to write a blog post that I can refer to.

Thank you for your time