First plan your trip. Know everything so you don't have to worry about any possible scenarios. Here are some questions that you should answer to help go:
1) How are you going to get there? Airplane or are you driving? If flying what is the cost of round trip airfare, and if driving, what would it cost to rent a car and then cost for gas? Here is a link that will help you plan your transportation: http://www.travelocity.com/
2) How much food will you need for the 4 days that you will be staying, and how much will it all cost? Here is a website of food that is good for camping and how to cook it: http://www.lovetheoutdoors.com/camping/camp_cooking_tips.htm
3) Where in Yellowstone do you want to go? Make an agenda of the things you would like to do and see. Here is a link that can help you navagate the terrain and get an understanding of the area: http://www.yellowstone.net/maps.
4) What will you do in the event of an emergency? Plan for those scenarios that may occur. For example, while you are out on a hike, your friend trips and falls and cuts their arm open on a rock. How would you take care of it? Pack a First Aid Kit to help you while you are there. Here is a link that will help you with your first aid: http://www.mayoclinic.com/health/FirstAidIndex/FirstAidIndex
5) Which of the 7 principles are you going to have to work hardest to follow? Here is the link to the LNT main website for help: http://www.zunal.com