Israel is a modern, developed country, and you can purchase virtually anything you need during your stay, including clothing, cosmetics, and hygiene products.
If you are visiting Israel during the summer you will need lightweight clothing - short-sleeved and sleeveless shirts, shorts, sandals, beach shoes and a bathing suit.  It’s also a good idea to pack a sweater or jacket, since nights in the mountains and the desert can be cool.
If you are visiting Israel in the winter, you will need warm clothing, a coat (preferably a raincoat as well), good shoes, an umbrella, gloves, a scarf and other warm clothing.  Weather in Israel is not cold as it is in Europe, but days can be rainy and cold.
It’s a good idea to bring a small bag for day trips.  If you are traveling to Eilat or the Dead Sea, it’s a good idea to bring a bathing suit, since it is warm enough to swim there even in the winter.

Sunscreen, a sun hat, and sunglasses are essential items throughout the year.
If you are planning on hiking, you will need good shoes and a lot of water, either in a canteen or in several bottles. You will need a sleeping bag, tent, and camping equipment only if you are planning on sleeping outdoors.  Most youth hostels supply sheets and blankets.