Little assistance with my tic-tac-toe program

I need some help with my tic-tac-toe game that I created in Python 3. Have a look at my fun program and try it out. After that, please help me creating a while statement in my program. That is while the user choice of square if filled, you should continue to ask them until they choose an empty square. When the choose an empty square, the program continues as before. I am not used to the while statements, please help me on this!

Here is my program:

from turtle import *

def setUp():
#Set up the screen and turtle
    win = Screen()
    tic = Turtle()
    tic.speed(10)
#Change the coordinates to make it easier to tranlate moves to screen coordinates:
    win.setworldcoordinates(-0.5,-0.5,3.5, 3.5)

#Draw the vertical bars of the game board:
    for i in range(1,3):
       tic.up()
       tic.goto(0,i)
       tic.down()
       tic.forward(3)

#Draw the horizontal bars of the game board:
    tic.left(90)    #Point the turtle in the right direction before drawing
    for i in range(1,3):
        tic.up()
        tic.goto(i,0)
        tic.down()
        tic.forward(3)

    tic.up()        #Don't need to draw any more lines, so, keep pen up

#Set up board:
    board = [["","",""],["","",""],["","",""]]

    return(win,tic,board)

def playGame(tic,board):
#Ask the user for the first 8 moves, alternating between the players X and O:
    for i in range(4):
        x,y = eval(input("Enter x, y coordinates for X's move: "))
        tic.goto(x+.25,y+.25)
        tic.write("X",font=('Arial', 90, 'normal'))
        board[x][y] = "X"

    x,y = eval(input("Enter x, y coordinates for O's move: "))                 
    tic.goto(x+.25,y+.25)
    tic.write("O",font=('Arial', 90, 'normal'))
    board[x][y] = "O"

# The ninth move:
    x,y = eval(input("Enter x, y coordinates for X's move: "))
    tic.goto(x+.25,y+.25)
    tic.write("X",font=('Arial', 90, 'normal'))
    board[x][y] = "X"

def checkWinner(board):
    for x in range(3):
        if board[x][0] != "" and (board[x][0] == board[x][1] == board[x][2]):
            return(board[x][0])  #we have a non-empty row that's identical
    for y in range(3):
        if board[0][y] != "" and (board[0][y] == board[1][y] == board[2][y]):
            return(board[0][y])  #we have a non-empty column that's identical
    if board[0][0] != "" and (board[0][0] == board[1][1] == board[2][2]):
        return(board[0][0])
    if board[2][0] != "" and (board[2][0] == board[1][1] == board[2][0]):
        return(board[2][0])   
    return("No winner")

def cleanUp(tic,win):
#Display an ending message: 
    tic.goto(-0.25,-0.25)
    tic.write("Thank you for playing!",font=('Arial', 20, 'normal'))

    win.exitonclick()#Closes the graphics window when mouse is clicked


def main():
    win,tic,board = setUp()   #Set up the window and game board
    playGame(tic,board)       #Ask the user for the moves and display
    print("\nThe winner is", checkWinner(board))  #Check for winner
    cleanUp(tic,win)    #Display end message and close window


main()