Trust experienced hikers to give you the advice that will provide you with the most comfortable hiking experience.
A good backpack is make or break for a comfortable trek. A backpack should have adjustable straps around the hips and chest. "Carry" your backpack with your hips, and not your shoulders - this will save you a lot of energy and potential pain!
Some days will be cold, and at night you might even experience tempratures below zero. Use 100% pure wool garments next to your body. It isolates and warms even when wet. Alternatively, used a combined synthetic wool blend if you sweat a lot. Cotton dries slowly and does neither warm or isolate, and is not recommended.
For those warm days, choose synthetic textiles (like dri-fit from Nike) for tops and thin ripstop nylon for trousers. These materials weigh very little and dry very quickly. On cold days, wool long-johns will keep you warm even if your trousers are thin.
Good hiking boots are essential for a good experience. Make sure they are well worn in so that you don't get blisters, and that they have good ankle support. Vibram soles or something that grips well on wet rocks will give you a confident stride.
A little duct tape rolled up can be a life saver in many cases. It can be used as emergency blister aid, for taping small cuts, fixing holes in jackets, back packs and shoes and mending broken walking sticks.
Bring a waterbottle that can handle warm beverages. At the cold campsites, ask for hot water in your bottle before you go to sleep. Put the bottle in your sleeping bag by your feet for a warm and cozy night. In the morning you will have water for your trek. Don't forget to tighten the lid properly!
Pack a very lightweight down jacket. Down is incredibly isolating, but will not make you feel clammy, and takes very little space and weighs practically nothing. Perfect for nights in the camp, but also for stuffing under your sleeping bag as a soft pillow.