Your listing's map location is automatically determined based on the listing address you provided.
You can add/change the address when you create/edit the listing.
Note that when keying in your address, a dropdown menu appears with the closest match available - you will have to select an option from that list. Otherwise, your listing may not appear on the map.
You may find the following articles useful:
How do I create a listing?
How do I edit my listing details?