First, a few rules for being outdoors.
- Avoid cotton. Cotton, when wet, stays wet a very long time. If you are in a wet situation and stay wet, you can develop hypothermia.
- Over prepare. It doesn't matter if the weather forecast says it will be bright and sunny, prepare for weather changes. You can keep the extra layers in your backpack.
- Don't over do it. Being too warm is just as bad as being too cool.
Base layer
Your base layer is the layer that touches your skin. Its purpose is to wick away moisture and regulate your body heat. It keeps you cool in the summer and warm in the winter. Look for long sleeves and legs made of merino wool or synthetic materials specifically made to wick away moisture. I prefer wool as it is comfortable and is naturally odor resistant.
SmartWool makes great base layers.
Mid layer
The middle layer provides your first layer of insulation. It is what consists of normal casual wear, though it should be functional for outdoor use. Again, stick to wool or synthetic materials. Flannel wool is a good shirt. Find one with chest pockets, as they will get used. Pants are meant to get beat up, so a synthetic material may last longer than wool in this case. I like the convertible pants which allow for even more layering options as the heat rises.
I could do a whole article on socks. Again, wool or synthetic socks. The thickness of your socks will be determined by the season, and how much weigh you will carry in your backpack. SmartWool is the go-to for many adventurous types.
Outer layer
The outer layer provides the greatest amount of insulation, for the coldest times of the day. This will adjust with the seasons. For the warmer months, it can be as little as a thick sweater. As it gets colder, you will add a thick jacket. I like the idea of having a two part outer layer, with a down vest and a jacket. The down vest will keep your core warm, and when the cold begins to bite, add the extra layer provided by the jacket.
Shell layer
The shell layer provides protection from the wind, rain and snow. Some people will combine the outer and shell layers, and that's fine. But for more flexibility throughout the year, adding a poncho and snow pants is a good idea. This will keep you dry, and keep the bitter wind from penetrating your layers.
Boots and head coverings
Finally, you will need boots that will keep your feet comfortable, dry and provide support for climbing up and down hills with weight on your back. This can take a whole article in itself, so I will simply refer you to your local outdoors specialist (like REI or Bass Pro Shop) to get yourself fitted. You will need to wear socks of the appropriate thickness while trying on boots. Consider how much weight you will be carrying, and what kind of terrain you will meet.
They say your head lets the most body heat escape. Whether that's true or not, it is important to keep your head warm. A simple wool cap can keep your head and ears warm, or opt for a balaclava which you can wear as a wool cap, and then pull down to keep your nose and face warm too. In the summer months, a wide-brimmed hat is ideal to keep the sun off.