This book is an excellent resource written by someone who has many years in this field, as well as a PhD in animal behavior. It is intended for those who either are considering becoming part of an established organization's efforts, or may seek to establish one themselves.
It is critical to understand that it is not just about removing horses from abusive or neglectful situations. It is necessary to know how to support them in all aspects so they do not fall through the cracks again. It is also imperative to know how to appropriately care for horses who have been abused, malnourished, etc. - as well as when to know when even the best care will only prolong suffering.
I am not sure from your post whether you are old enough to legally become involved, but I encourage you to volunteer your efforts with a legitimate, well-established organization such as Bluebonnet if you are interested in helping equines in need.
What state are you in? Your title would make a lot more sense if you simply stated you are looking to help some very underweight horses in your area. Tell us where you are so we can help you. There are MANY laws that actually prevent AC from doing things in certain situations. We would need a lot more details, who have you already contacted?