One of the essentials for any hike is a bottle of WATER! You have to keep yourself hydrated and bring plenty of water when you go. Especially in summer, with the dry weather, at least two quarts per day is recommended.
Another essential is wearing the appropriate gear, and shoes are one of the most important items to consider. Long sleeve t-shirts and long pants are a great option for apparel. For shoes try using hiking shoes, stay away from flip flops and looser shoes!
The last essential is to bring some granola bars or energy bars. While hiking, you will need a lot of energy, and a good way to keep your energy up while hiking is to eat an energy bar. They are also convenient because you can keep them anywhere with little extra weight.