It depends, do you want to be nice about it or not? ^_^ Also, here in America, if you tell your boss that you are leaving your job before you find another job, they might let you go before you can start your new job. Usually you would wait until you have another job with a starting date and then give your employer your 2 weeks notice.
"I am currently looking to change careers, so please keep this in mind as i may be leaving this company soon." (Makes them feel like you are not leaving your job because there is a problem but because you are looking for something different)