It's probably a good idea to check out the online tutorial on the main menu, and then read through some of the game's manual. It takes some time and patience to learn, but once you grasp the basics it's pretty easy to start picking up speed! For starters, here's a classic introductory program you can try:
Type this in EDIT mode (Minus the comments after ')
CLS 'This command clears all text from the screen.
@LOOP 'This acts as a marker, and we can make the code come back here later
PRINT "HELLO WORLD!" 'This will make the words in the quotes appear on screen
GOTO @____ 'Fill in the blank! You want this command to return to the @ earlier in the code!
When you return to RUN mode, type RUN and press ENTER. This will run the code you entered in EDIT mode. If you did it right, the screen should fill up with "HELLO WORLD!". What's happening is the code first reaches the command to clear the screen, then it reaches a command telling it to PRINT the text, then it reaches a command telling it to return to the line with the desired @ label.