If you have just purchased your Apple TV, I'd assume that it's the third generation, or in rare cases, the second-generation.
Now to your question. If the apple TV buffers for a long time, press the "menu" and "down" button and hold it for 3-4 seconds to perform a restart. A guide to restarting your Apple TV is here : http://support.apple.com/kb/HT3180
Try checking your internet speed too. A slow internet connection may result in long buffering time for movies. Try connecting your Apple TV to the router in your house by Ethernet (LAN) cables.
However, if you still encountering this problem, even after trying to change your internet connection to Ethernet, or even switching your internet service plan for a faster connection, you'll need to perform a factory reset of your Apple TV. Follow this guide on how to restore your Apple TV.