What is the problem and what can I do to fix it?
More information would be very helpful.
When you turn the ignition switch, what, exactly, happens? Do you hear nothing, or do you hear clicking, or something else? Does the starter turn the engine? If so, does it turn vigorously or weakly? Does it start sometimes, or never? How long has this been happening? What’s the maintenance history of this vehicle? How old is the battery?
“Won’t start” is too generic. We need details.