First off you need to be rooted.
If you're on a stock rom, you more than likely have to download a screenshot app from the Play store.
If you're on an MIUI rom it is vol down+ menu button
If it's on a CM rom you can hold the power button and the power menu should pop up and give you an option for screenshotting. I never really tried any of CM roms, usually stuck to MIUI, so not sure if there's an easier way.