Welcome to the world of work! May you always have three job offers to choose from!
Seriously, in the 21st century (in the USA anyhow) it's really rare to hear back on a job application unless the employer wants to interview you or hire you. An old guy like me finds this quite disturbing: it's a small world and it's not smart to ignore people. But, it's still the reality.
So, apply for plenty of jobs. If you know somebody in the workplace ask their advice on how to apply. If you haven't heard back on an application, you can assume they aren't interested.
A job offer is a conversation, letter, or a phone call that informs you where and when to show up, what you'll be paid, who your supervisor is, and how to contact your supervisor. It also may say other things about the workplace. If you're in the USA it will probably tell you to bring along two kinds of identification so you can fill out an official form called an I-9.
My point is this; you'll know when you have a real job offer. If you're not sure whether it's real, ask. If there's nobody to ask, the offer's not real.
It's fine to have more than one job offer. As somebody else mentioned, it's fine to ask for a day or two to think about an offer before accepting. They aren't going to withdraw the offer if you ask for that, but they may say, "no, we need to know now." In that case, you need to decide.
If it's a summer job, stick with the first job you accepted unless you get offered another one that's solid and pays at least 20% more. Everybody understands moving up from minimum wage to something better.