To insert text you must first enter insert mode. The most basic way to enter insert mode is with the i (insert) command. You will notice you are in insert mode when the caret changes from a block caret to a thin caret.
When you are done inserting text, press the ⎋ (escape) key to go back to normal mode.
The i command enters insert mode at the current location. To enter insert mode after the current location, you use the a (append) command.
If you want to append text to the end of the current line, no matter where on the line you are, use the A command.
Similarly, the I command moves to the first non-blank character on the current line and enters insert mode there.