! Heading 1
!! Heading 2
!!! Heading 3
!!!! Heading 4
!!!!! Heading 5
!!!!!! Heading 6

Headings should be used sparingly in TiddlyWiki. If you find yourself creating complex multi-level headings within a tiddler, you should probably be creating multiple tiddlers instead.

Heading 1 is usually not used (in HTML documents, it is reserved for the page title).

Anything below Heading 4 is smaller than the body text and not particularly useful. If you find yourself needing anything deeper than Heading 3, you should definitely be splitting out into multiple tiddlers anyway. I typically try to avoid more than one level of headings within an individual tiddler.

Headings are block-level elements, so you need to put a blank line between text and a following heading, or the heading will get rolled into the preceding paragraph, which is almost certainly not what you want:

Here's an example of what not to do:
!! Heading 2