A good rucksack is a must for me - waterproof, not too big or heavy, needn't be expensive but comfort is a must. Rubbing across shoulders gets painful.
What you carry depends on when, where and how far you are planning to walk but I often carry some water/food, map/guides, glasses (to see the map with 🤣) and/or sunglasses, lip salve/sun cream (depending on weather and length of walk), small box of plasters, phone for emergencies, house keys, car keys, gloves and hats that are take off en route. Much easier than having pockets loaded down with stuff and I like to keep my hands free.