Incorrect: I come from Italy. No, this is correct. It either means that you are a native of Italy, or that you recently journeyed from Italy.
Correct : I am from Italy. (reason??) The most common use here means that this person was born in Italy.
Incorrect:I am in Bahamas since last week.
Correct: I have been here since last week.
Incorrect: Complete your work 'I did my job. This could be correct by inserting a comma - "Complete your work, I did my job". Here you are telling someone to finish their job and that you have finished yours.
correct: I have already done my job. (Reason) Another way of saying that I have finished my task.