See our page on boots for more advice. See our page on boots for more sock advice. They hold water when wet, allow cold air reach the skin and encourage hypothermia. Cotton are just as bad. Manmade fibre are ideal because they don't soak up water and dry quickly.
Breathable underwear is best.
Always best to ask other walkers what they like and why. Decide if you are going to using it day walks or for a longer period. Make sure you try it on and fasten all the straps and belts. This prevents the contents of the rucksack liner becoming soaked if a bottle or flask should break. It keeps your 'bum' warm and dry when you sit to eat your snack.