Vim FAQ: What is the Vim insert command?
Short answer: Type the letter 'i' while in Vim command mode. You'll then be able to type text into your document, right where the cursor is currently positioned.
Vim insert commands
Longer answer: There are several other variations of the Vim insert command you can use:
i - insert text at the current cursor position I - insert text at the beginning of the current line a - append text after the current cursor position A - append text at the end of the current line o - open a line below the current line O - open a line above the current line
You'll want to issue all these Vim insert commands while in Vim command mode.