You are human and will remain human. There is no spell to make you a demon. There are many spells to summon demons, though I doubt any are easy. They all require work, attention, and enough knowledge to get it done. Also the first thing one should learn about summoning demons is banishing them, never call out what you can't put back is more than just good adivce.
How exactly do you define a demon spell? I believe you should start with something less complicated. THis can be very dangerour for a begginner. Otherwise it won't even work. I recommend you to try and expand your knowledge rather than looking for power. Besides I m sure that the power you have in mind is not something you will get here, otr anywhere else.