Your sleeping bag will take up a relatively large portion of your pack, so it deserves special attention. Some backpacks come with a separate compartment for sleeping bag storage. Those packs are an excellent choice because the separate compartment holds the weight of your gear off of your sleeping bag. When this is the case, you'll get better performance from your sleeping bag, particularly in cold weather. Sleeping bags, like other insulating gear, keep you warm by trapping air inside the fill. When the fill becomes compressed, the sleeping bag can't insulate as well. With that in mind, you may wonder about the wisdom of using a compression bag for your sleeping bag.
A compression bag is used to reduce the bulk of your sleeping bag, freeing up room in your backpack. Because you roll up your sleeping bag before putting it into the bag and compress it evenly on all sides, a compression bag will not cause the same problems that packing heavy items on top of a sleeping bag will. However, it's important to compress the bag evenly and not tighten the straps too aggressively to prevent damage to your bag.
Besides your sleep bag, you'll also carry a sleeping pad. There are several ways to pack your sleeping pad. Some people fold it and pack it on the side closest to their backs. Packed against the back, the sleeping pad provides some protection against sharp cookware or other items that may poke you as you hike. If the sleeping pad doesn't fit easily into your backpack, it can be attached firmly to the outside, along the back or between the front of the backpack and your body. If you attach it on the sides, it's more likely to become tangled or snagged when you pass trees and bushes.
As far as clothes are concerned -- how much you pack will depend on how long you'll be out backpacking. Let's say you're packing for a five-day trip. Generally speaking, you'll want -- at least -- a complete change of clothes and something to sleep in. You'll need at least one, and maybe two, extra sets of underwear. Microfiber blend clothing dries quickly, which allows you to rinse it out and have it dry by the next morning. Three pair of socks -- one to wash, one to wear and one pair that's dry -- should be enough.
Of course this is a simplistic list, and you'll need to consider the weather conditions. In the summer, in mild conditions, one complete change of clothes may be sufficient. Add a cotton or fleece hooded sweatshirt for early mornings, and you're ready to go. Realistically, however, plan on carrying rain gear, long and short sleeve layers, a hat and gloves, and even long underwear. Plan on getting colder, hotter and wetter than you imagined. Although a good deal of variety isn't necessary, clothes that offer flexibility are.