@Shawna40
Hello Shawna, it is a pity you have gone 'live' before you have properly set important details of your listing.
Firstly to prevent any more damage go to your listing editing page and follow these screenshots...
Then once you have done that scroll on down until you come to the extra charges section and this is where you will put in the extra guest charge that you wish to charge for all guests after your base number.
Remember to click 'Save Changes' once you have your guest setting correct!
Do this immeditaley and at least you will not get any further bookings that you will have to honour!
I would suggest you contact Airbnb and ask if there is some way they can help you with this existing reservation.
You can't ask the guest for more money and the guest is not obliged to pay you more, and if the guest does not wish to cancel, then you are either going to have to cancel yourself and accept the penalties or host this reservation at a loss.
Remember @Shawna40 if you cancel a guest reservation the consequences will be considerable. It will affect your future bookings for years to come....prospective guests do not like booking with a host who has a record of cancelling reservations, and the record will show on your profile and listing......it give them the jitters and they book elsewhere. So it will not just cost you now, it will cost you long into the future.
I would suggest that as a gesture of goodwill and as part of your learning curve you just accept this current reservation, explain to the guest you are new and made a mistake but you will honour the reservation as a matter of principal. At least that way youi may end up with a good review....and that is something!
Cheers.....Rob