It is very difficult for me to imagine if I can survive without any clothes, tools etc with an unknown partner for 21 days as the current life is full of tools and very convenient.
Anyway, imagining I am in a forest, I will look for the place that I can stay safety and comfortably first. It should be like a cave on a higher place to avoid the attack by foreign enemies. In addition, I learned from the book that smells are lost in water. So, it is better to near river or lake.
Secondly, I will find something that I can eat. I like fruits, so I would like to get them as many as possible. However, it will be not enough to survive for 21 days. Therefore, I will try to catch fishes in river. I prefer to meets, but I am not sure I can kill animals. Moreover, I will have to build a fire to grill the fishes. It should be very tough job.
Finally, I want to find a friendly partner. If I have a partner like monkey, I can learn about the forest more and must be encouraged in the life without people.