1. Soap, and clean drinking water (in a canteen), though hopefully you are camping near fresh water of some kind (just remember to boil!).
2. First aid kit. (A small one is normally fine, but sometimes I add an emergency blanket or other small trinkets depending on weather/climate.)
3. Small condiment packets, such as sugar, ketchup, etc. if you wish (Be sure to plan your meals).
4. Layers of clothing, depending on weather/climate you are going to.
5. Good, sturdy boots, preferably above ankle.
The most important thing, though, is to invest in your camping gear. Your tent, sleeping bag, etc. can all be bought for cheap, but I've found that with the essential gear, it's worth spending the money for the good stuff. My husband and I used to use regular Walmart brand sleeping bags, but once we bought $100+ sleeping bags that were rated for negative temperatures, we've never gone back. Even in the summer when you don't need the heat, they make great padding because they are so well made. Hope this helps.