It depends how you want it to work. Probably the simplest is to make one global variable and switch that between values
Global variable = 0
Global variable = 1
Changing turns can be made by timers - if some time passed set global variable to 1 or 0 or 1-global variable (will switch automatically between 1 and 0)
or player movement - if player finish moving - end turn
If I remember correctly there should be an example of turn based games in "How do I" - sticky post, but it's lost somewhere due to new forum installed recently