"I want to be a singer."
Translation:Şarkıcı olmak isterim.
I would translate the first as "I want to be a singer" and the 2nd as "I would like to be a singer". So the more permanent/decided option is actually the "istiyorum" one.
In daily speech, the "-r" tense is rarely used in personal wishes; I think the sentence with istiyorum makes for a better standard answer.