Ok. With the xbox you will recieve an ethernet cable. This plugs into the back of the xbox and will go into your laptop. This will share the internet connection from the mac to the xbox. Thats extremely simplafied, and often doesnt work out so easy, but it did for me!
You will also need a gold xbox live membership to play online. When you buy the xbox you should get a free 1 month trial which im currently still using. After that you will need to purchase and xbox live subscription for around £40.
Hope this helps