Don't make a big production out of it. If you want to leave, you can leave. You're "at will," which means you're only going to work if you want to. Don't worry about what you'll say, it doesn't matter. You don't owe this company lifetime service (and they don't owe you a lifetime job). If it was in their best interest to terminate you, they would do so promptly. If it's in your best interest to leave for something else, you have the same rights.
If they terminated you, you would want them to treat you with respect. Do the same for them if you want to leave for a different job. Keep in mind that it's a small world, and you never know when you might run into these people again. Nobody's going to disrespect you for trying to better yourself, but they will if you create a bunch of drama. If you can comfortably give a two-week notice, do it. If you have to leave immediately, do it. Just be straightforward and honest. You don't owe anyone any explanation, but again, it's always best to leave with the best relationships possible with everybody. You might meet them again sometime.