Correct spelling, explanation: although pronunciation may seem to point to amature or something similar, amateur is the correct word. This is due to the fact, that this word comes from French amator, and the suffix -eur is often used in loanwords from French (another example of this usage is entrepreneur). Therefore the correct spelling here is amateur.

Definition of amateur:
noun, somebody who does something unprofessionally; someone who doesn’t take money for what they do, a fan or unskilled person
My uncle used to be a fishing amateur, but he doesn’t fish anymore.
Can you imagine that he isn’t a professional pianist? He is an amateur, but he is very talented.

Incorrect spelling, explanation: this spelling is incorrect and probably arose due to slight mispronunciation (the correct is /ˈæmətə(r)/). Amateur derives from French amator and as with many other French-originated words it has the suffix -eur. Therefore amateur is the correct spelling.

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling

Incorrect spelling