When it comes time to choose the proper hill walking equipment you need to keep in mind what season it is, and the level of hill walking that you intend to participate in. However, there are general items that you should carry with you in all weather conditions outside of winter and on most levels.
The best place to begin is with your clothing choices, which should include both warm mid-layers and waterproof outer layers. You also need to remember your lower body and make sure that your legs are also protected in weatherproof clothing.
Additionally, you need to carry spare clothes in the case of sudden changes in the weather or in the case that an emergency occurs and you need to set up camp. You also will need to choose a pair of shoes that are both comfortable and supportive since you will be placing a large amount of strain on your feet.
Other items that you will need to carry with you while hill walking include a torch to light the way, a whistle to stay in contact with your group, and a map and compass for navigational purposes.
You will also need to ensure that you take an adequate amount of food and water with you since it is easy to get dehydrated in both cold and hot weather due to the amount of physical strength it can take to get up a mountainside. In cold weather you may want to bring a hot drink inside of a flask. You should also pack extra high energy food so that if you need to set up camp you have an emergency supply of food.
Unless it is the middle of summer you should also pack a pair of gloves and a hat since the weather can shift dramatically along mountains and hillsides. Additionally, you should pack re-sealable plastic bags so that you can place your equipment inside in the case of rain.
As you never know when a storm may set in over the hilltop you will also want to pack a survival bag or some type of bivvy shelter that can be set up if you are on a day trip and there is a reasonable chance that you will be caught on the mountainside overnight.
There are also a few seasonable items that you may want to pack depending on the time of year, in particular the summer that include insect repellent, sunglasses, and sun block as even under tree cover you will still be exposed to the ultraviolet rays from the sun.
Other items that you may want to consider bringing with you are a mobile phone so that if you get lost or an injury occurs you can call for help, GPS receivers which will help you navigate better than a compass, and a camera so that you can document your hill walk. It may also be helpful to bring walking poles to take the strain off of your legs.