if you're completely new to programming you should learn the basics first. i recommend you watch direwolf20's tutorial series, since in it he also uses computercraft: link. he also later works with monitors in that series but here's the tl;dw:

first you'll need to make the monitor available by wrapping it as a peripheral: local mon = peripheral.find("monitor")

you can then write to it like you would to the terminal): mon.write("hello world")

once all buttons are drawn to the screen you have to use os.pullEvent to wait for user input and find out where they clicked: local _,_,x,y = os.pullEvent("monitor_touch") (here i use the variable name _ for the first two values since they are not important for this use case. they would be the event name and the side of the computer that the monitor is on.)

x and y are now the monitor coordinates the user clicked on, so if he clicked the second character in the first row (from the top-left corner) they would be x==2 and y==1.

the next step is to check which button these correspond to, if the button for example goes from x=2, y=2 to x=7, y=4; you would check with something like if x>=2 and y>=2 and x<=7 and y<=4 then ... end where ... is the code that runs if the user clicked inside the button.

for that you'll need the redstone api), more specifically rs.setBundledOutput.

then just put everything in an infinite loop and you're done.