When buying a backpacking sleeping bag, you often have the option of having the zipper on the left or right side of the bag. When you're lying flat on your back in a sleeping bag, a left zipper sleeping bag will zip up on your left side, and a right zipper sleeping bag with zip up on your right.
This makes the left zip sleeping bag, easier to zip up if your right-handed and a right zip sleeping bag easier to zip if your a left-hander. You'll essentially want the zipper on the opposite side of your dominate hand.
So if your right-handed, you'll likely want the zipper on the left side of the sleeping bag because when you're lying flat on your back, it's easier to unzip a sleeping bag with your right-hand if the zipper is on the left side of the sleeping bag. See: Left Zip Sleeping Bags
If your left-handed, you'll probably want the zipper on the right side of the bag. See: Right Zip Sleeping Bags
However, if you're a side sleeping and typically sleep on your right side, you may consider getting a zipper on the right. But more than likely, if your right-handed you'll still want the zipper on the left side of a sleeping bag.
But more importantly, it's not what side the zipper is on, but how warm the sleeping bag is going to keep you thoughout the night. I'd recommend you buy a sleeping bag rated for 15 degrees warmer than the temperatures you expect to be sleeping in if you're going to be camping in near or below freezing temperatures. So if you plan to sleep in temperatures of 30 F degrees, you'll want a sleeping bag rated to keep you "warm" to at least 15 F degrees. Remember, you can always unzip the bag if you get too warm.