How to put shelves up using only finishing nails?

i have a set of 4 shelves that i had gt a long time ago. I now know where i want to put them but the problem is that the mounting hardware that was supposed to come with it is missing from the pack and all i have on hand are some finishing nails. I tried using two nails in each hole since the shelves already have the same type of fixture. so i tried attaching the shelves to the nails but it would sag and the nails would get pushed in as i would try to attach the shelves. Im really frustrated right now as i want to use them to organize but i dont know what else to do as this is my very first "remodeling" type project. Im also a novice beginner and have had no prior experience with anything construction related. I also have to get this done fast as i have guest coming soon and only have two days to complete the shelves and get everything organiized and ready. PLEASE HELP ME!!!!!!!!!!!!!!!!!!!!!!!