You might only have a single H1 on a page. This is a normal paragraph of text inside a <p> tag. Keep the text in the heading free from other tags, such as underline <u>, bold <b>, italic <i>, or ≷strong>so that the headings have a uniform appearance.
If the obvious standard style to use looks ugly or wrong, we should fix it in the style sheet so that it does the right thing consistently through the site rather than using odd styles.
Use H2 for subsections. You can have other sections, H3 and H4 and so on. It would be good style to progress through the headings rather than, say, jump straight to H3 below an H1.
If you would like a line to divide sections use a <hr> tag...
Ordered lists are numbered, Unordered lists have bullets. Both lists can have multiple levels.
Both lists use the <li> tag round the items in the list.
These use the <ol> tag. For example
These use the <ul> tag. For example