It sounds like you want the Dialog function.
In the trigger editor, take a look at Dialog. (You can use the trigger-search functions)
You can make a little menu appear with buttons. You have to assign variabels to the buttons, (Crt-X in Trigger editor) then use the Event - Dialog Button Is Clicked and then it'll run that trigger, in your case being the starting units.
Example, off the top of my head:
Create Dialog
Set Dialog Title = "Pick A Race"
Add Dialog Button to LastCreatedDialog with text "Orc"
set Button1 to LastCreatedDialogButton..
Add Dialog button to text "Human"
set Button1 to LastCreatedDialogButton..
etc...
Show LastCreated Dialog for the player.
Then create triggers, with the event
If button1 is clicked..
Do your stuff.s