On my first spell, a lucky charm (which failed) I was a bit nervous because it was my first attempt. I didn't even know the basics neither if the spell was correct. I felt that because I didn't know what to expect, I'm not sure if had even understand how magick works, back then. Later I discovered how it works and that I should start from the basics in order to cast spells succesfully. I don't cast spells though...but they're needed for anything related to magic. You can avoid back-fires if you use the basics properly. You just have to become familiar with it and realize if you use magic properly, nothing bad is going to happen to you.
About spirits as Risha said, ks the fear of the uknown. It's logical to be afraid to see them, they're non-physical entities... I used to be afraid of them when I was younger, about 3-4 years ago I was reading stories on internet about people using Ouijas, contacting them and I was very scared..I was even scared if a spirit was around me. As years were passing I learnt more and I became familiar with them... I haven't seen one yet, but they don't scare me (I'm not sure of my reaction when I see them, but I don't think it'll be negative...). I realized they are souls like us and sooner or later you'll be one too... Of course there are spirits who will try to scare you...but that shouldn't hold you back, spirit world is amazing and there are entities that want to help and support you... Now, I'm dying to use a Ouija and if I read stories on internet I won't get afraid, but excited.
That's how I have overcome my fear.