You will have to purchase one of two devices to allow the Apple TV to connect to the internet.

Either purchase a Wireless adapter to allow you to network the Apple TV to the internet through the laptop. ( This will require the laptop to be powered on to have the Apple get internet. Your laptop and the Apple TV will be sharing the same connection, so you could have severe lag at times.)

OR

Purchase a rather low cost wireless router to place between the laptop and the modem. This while a little more expensive than the adapter method, has two major advantages.
1. You do not have to have the laptop on, or even connected to the network for the Apple TV to gain internet access.
2. The router will allow both the Apple TV and the laptop to have full access to the internet, rather than sharing a connection.