1) Buy an Apple Airport Express, and set it up, you only need to do this once and it only takes a couple of minutes.
2) Plug in your airport express and connect your amp. or speakers to it.
3) Turn on your airport on your MacBook and connect to the Airport Express network.
4) Launch iTunes. On the lower tool bar you should see an option to play your music either via computer speakers or via the network that you set up in stage 1, select the option to play via your network.
5) Play and enjoy!