new to fly

i'm really new to fly stroke,i don't have a coach,but have watched others doing fly,when i tried i could do it but seemed to take loads of strokes per length,also it tired me to do just 25m,i can do 2500 breastroke without stopping,maybe about 100 free before getting very tired,any suggestions ,workouts with weights etc to assist me to getting better.perhaps a simple explanation of the fly stroke would be of help too.