Never, ever, announce that you are leaving a company before you have the new job, and the new start date, secured.
Announcing that you are interviewing elsewhere is a surefire way to be fired before you have your next job in hand.
Wait until you have signed your offer letter at your new company.
However, it is good practice to make sure that once you have secured the new job, that you make sure you can give a full two-weeks notice to your current employer. Generally, your new company will work with you on this, and your current company will appreciate that you at least gave a two-week notice, even if they don't keep you for the full two weeks.