Okay, everyone else has answered, but I will too. Cut the top bit off—just the very top so the each clove is a wee bit open to the elements. Drizzle some olive oil on it. Put the entire thing in some aluminum foil (this will save your oven and make it easier to grab later when the garlic becomes squishy). The aluminum should be shaped to make a little stand that holds the garlic up, catches the oil, but also allows the top of the garlic to brown.
Toss it in the oven for an hour or so on whatever rack, really at whatever temperature you need the oven on anyway, though I wouldn’t go above 400 or below 300. It should take about an hour to roast—faster if you’ve got the oven much hotter, slower if it is cooler. You’ll know it is done when the garlic is golden brown and squishy.
To get the garlic out of the cloves, I usually simply turn it upside down (cut side facing down) and squeeze. Squish! liquid garlic spurts out (more or less).