One mistake is all it takes. If summoning is anything like I understand it to be, you could open the possibility for demons or other dark entities to attack you. Not in a physical sense. But if done incorrectly, or without proper precautions, summoning can bring a lot of quite negative things with it. It depends on the specifics of the summoning you're doing, but if it's calling out to a spirit or something similar, there's a reasonable chance some darker energies will come flocking to your call as well. And they don't just want to chat would be my guess.
But as others have said, such things are far too complex for a beginner. You have to understand how these things work, and the basics behind them. You cannot just simply go out and do it. That's a good way not only to fail, but to even hurt yourself. You won't get anywhere without knowing the basics first. I advise you take the advice of others, and study them first.