It's pretty simple. You need to use the command msgbox.
If you have a small text u can put it in the braces like:
msgbox("Hey there", MSGBOX_AUTOCLOSE)
waitmessage
If the text should be longer you need to make a text script like:
msgbox(scriptname, MSGBOX_AUTOCLOSE)
waitmessage
text scriptname{
"Hey there how are you?/n"
"The weather is nice!"
}
There are different types of msgboxes depending on the type u used. For example MSGBOX_AUTOCLOSE or MSGBOX_NPC and so on. It's pretty simple you can always look at existing codes in the game.