When you're editing cards, above the text box you'll often see a toolbar containing buttons and a menu.
Going from left to right...
The menu on the left lets you change the current paragraph into
or
If you have four or more headings in a card, then a table of contents will automatically be shown at the top of the card.
The big B and I will make any text you have selected bold or italic, respectively (and of course you can use both).
The button that's a big double quote...
You use the next two buttons to make lists. The first creates:...indents a whole paragraph like this (also known as blockquote), and also adds some vertical space from the paragraphs above and beneath.
The second creates lists...:
Next is the button for adding horizontal rules like this:
The last button lets you look at and edit the actual HTML of what's in the text box. (Most HTML is normally stripped out. Set a card's type to HTML to use any HTML you like.)