import Tkinter
root = Tkinter.Tk()
# first variant
chek_one = Tkinter.Checkbutton(root, text="I have been checked")
chek_one.pack()
chek_one.select()
#second variant
var = Tkinter.IntVar()
var.set(1)
chek_two = Tkinter.Checkbutton(root, text="I have been checked", variable=var)
chek_two.pack()
root.mainloop()