Hold the volume up and down buttons at the same time, and keep holding them down. During the time you're holding them down, after about 20 or so seconds, the screen will go off, you may see the red LED come on briefly, eventually you will see the "BlackBerry" name display for about 20 to 30 seconds. When that name disappears, then let go of the buttons.
From there, you'll see the logo with the sweeping boot up progress squircle. When that's done, it should be at the home screen and it's finished - the phone is ready.
The whole process should take maybe 2 to 2 and a half minutes.