Main titles will usualy make use of heading one

Subtitules and main sections might need to use h2

H3 usually finds its place on nested sections inside a bigger block

In case you need further sectioning, use an h4. You can go down any number of levels. Well, actually, not

Rarely you will need a heading five. But if you need it, you can use it. This is really nested and the font-size is very decreased

Heading six is rather unexpected. On this nesting level the font-size has dropped and is small, even compared with normal text