Correct: I've decided I'm going to look for a job. More normal: I've decided to look for a job.

for "going to" you need the conjugation of the verb "to be" "I am" going to look for a new job. "We are" going to look for new jobs. "He is" going to look for a new job. For will and shall (which are mostly interchangeable, but "will" is more natural and preferred) you do not need the "to be" verb. I will look for a new job. I shall look for a new job.