To sell online you need a payment gateway and a shopping cart that can interface with it. A payment gateway would be something like Paypal or WorldPay.
Your website would be coded so that people fill in their details on your website then they are transferred to the payment gateway to enter their payment details.
If your tickets would only be sold to people in the local area I would recommend using a reservation system where people can reserve tickets or seats and pay when they pick them up instead of making people pay for tickets online.
The coding would be easier and therefore cheaper.
If you'd like any more help feel free to contact me via PM.
To make peace you must be prepared for war