Hiking boots or shoes. These come in a variety of high top (better ankle support), or low top (more comfortable) styles. Some hikers wear various rugged outdoor sandals. Footwear should be rugged enough for the terrain envisioned. Hikers will generally consider water proofing the boots or shoes.
Pocket knife, possibly with a tin opener and a saw.
Flashlight plus spare batteries and bulb
Trail maps with sufficient detail to be meaningful
Water purification buying mineral water might be expensive, it will help you save some.
Water bottle – bring your water bottle, so that you can refill water in it through the plastic bottled mineral water, leave the plastic bottle in trash.
Plastic bags of various types and sizes to keep things dry and pack things out Garbage bags can be used to line the backpack with, but also to put in one's shoes to keep the feet warm, even when the socks are already wet.
Sleeping bag (and/or liner)
Clothes — best worn in layers, so one can easily adapt to changing circumstances. So two thin sweaters make more sense than one thick one. Also, on overnight trips, keep one set of clothes dry for evenings and nights (e.g. a jogging suit) and put the day clothes back on before one starts walking, even if they are wet.
A warm hat or cap — even when no cold weather is expected. Per weight and volume, this is the best insulator because a lot of body heat escapes through the head.
Socks Along with footwear most hikers should also consider socks that will help wick sweat from the hiker's feet, provide warmth, and provide buffering inside the shoe.
Towel — can double as a scarf or head dress (against the cold)
Snacks — preferably of the healthy kind, as emergency 'power food'.
Camera plus spare batteries and film/memory card