How would it be best to prepare yourself for a hunger strike?
Practice. Become familiar with what it feels like to be starving. Learn how to successfully deal with your mind, body, friends and family all conspiring to talk you out of it. Find out what a reasonable time goal is, which will require some research. There's a limit to how long you will survive, depending on your BMI, age, health, metabolism, etc. So make sure you plan to cut yourself off before you actually damage your health. If you can do that on a practice run, then when you have a real cause to risk your life for, you'll probably have the strength of body and spirit to pull it off.
That said, depending on the location and situation you might not get very far. In most of the developed countries, if someone objects and calls the authorities, they will come in and hold you down while two really big and strong paramedics inject you with a sedative, then you'll be locked up, immobilized and fed intravenously until you're no longer at risk, and then the authorities will select a long-term solution to the problem named Alex. The Russians have been known to just force-feed hunger strikers, which is extremely painful when the tissues of your digestive system have adapted to no food, and can cause more medical problems than you've already got from the starvation. In some countries they'd probably just let you die so somebody else can have your share of the food. Goddess knows what the Chinese would do and I don't really want to find out. In India they'd probably burn incense and start a fan club.
I'm not sure what you're planning to accomplish, but it's not likely to work. The world isn't what it was in the 1960s and we have to devise new ways to accomplish our goals.