At the top right of every card you'll typically see a card menu. To edit a card, click on "edit" in the menu, or double-click anywhere in the content of the card.
Wysiwyg
This is one of the best acronyms in all of geekdom: "What You See Is What You Get". Most cards have a "wysiwyg" editor, which basically looks something like this (learn about using the wysiwyg editor):
Some types, like Images, Files, Pointers and Dates, have different editors. If you edit an image or file, for example, you'll see a "Browse" button that lets you select a file on your computer.
Saving/cancelling and deleting cards
Underneath the editing area, you'll see "Save" and "Cancel" buttons. Click "Save" to save your edits, or "Cancel" if you want to leave the card as it was. If you have permission, you'll also see a button for deleting the card.
Linking and Inclusion
See links and nesting to learn how to insert them when you are editing cards.