Start with Crow pose (Bakasana) and Four Limb Staf pose (Chaturanga Dandasana) to develop your upper body strength and ability to balance on your hands. Then practice Tortoise pose (Kurmasana) to increase the lexibility of your hamstrings and back.
As you lift up into Firely, remember to squeeze your thighs firmly onto your upper arms before you attempt to straighten your legs.