Write a resignation letter. In many jobs, you’re supposed to put your resignation in writing anyway. Make it short and to the point. For example:
*Dear Boss Lady,
I regret that I must resign from my position at Charlie’s Chocolate Store. I have greatly enjoyed working here, and I thank you for the opportunity.
I am available to work my normal schedule until <date 2 weeks hence>.
(You don’t have to say anything about why you’re leaving, but a “position in my field” or “career opportunity” is always a valid reason to leave a retail job. )
Then ask your boss to speak with her privately. Hand her the letter, and let her read it. That’s probably the easiest way to “tell” her. If you can, offer to help ease the transition, like training new people, etc. Whatever you do, don’t feel bad about leaving because other people are. That’s just the way the cookie crumbles in the big bad world of retail.