Insurance 101
Insurance companies collect payments from customers in advance; these payments are known as premiums. When customers get injured or sick, or for any other reason need to see a medical professional or hospital, the insurance company pays for part or even all of the bill, depending on the arrangement they have with the customer. The process of getting the insurance company to pay the medical professional or hospital is called a claim. Sometimes the professional or hospital will file the claim on behalf of the customer; sometimes the customer files the claim.
Obviously for the insurance company to make money, the amount they collect in premiums has to be more than what they pay in claims. There are two main ways that insurance policies can be set up in order to reduce how much the insurance company pays: (1) there is a deductible amount, or (2) there is a co-pay amount. These are similar but not exactly the same, and sometimes both can apply to the same claim.
A co-pay amount is a fixed amount that the patient pays each time. For example, if your co-pay is $25, then every time you go to the doctor, be it for a regular checkup, because you're sick, whatever, you pay $25 and the insurance company pays the rest. In exchange, the premiums you pay monthly are lower. Lower, that is, compared to what you'd pay if you want the insurance to pay the entire bill.
A deductible amount is a fixed amount, much higher than a co-pay, that is your total responsibility to pay over a certain period of time (usually one year, but sometimes lifetime). For major things like a stay in the hospital, the insurance company will cover 80% of the bill, up to a certain amount. That sounds pretty good until your realize that even a few days in the hospital costs thousands of dollars. A serious injury or operation can cost tens or hundreds of thousands, so you're still on the hook for thousands, up to your deductible amount. If your total deductible is $10,000, then that's all you pay, period. No matter how high the bills go, the insurance company picks up the rest of the bill. This is to prevent people from getting completely wiped out financially by a single catastrophic illness. Yeah, $10,000 is still a lot of money, but you or your parents or hopefully somebody will be able to raise or borrow that much. A total hospital bill for half a million dollars would destroy a lot of people.
So... if your employer offers more than one insurance plan, then there's going to be a plan where you pay higher premiums but have a lower (or non-existant) co-pay. There will be a plan where the premiums are lower, but in return you pay more out-of-pocket each time you see the doctor. There can be any number of plans in between. There can also be plans with a higher deductible in exchange for lower premiums (think about it; it makes sense) and vice versa.
The company I work for offers I think seven different plans underwritten by three different insurance companies. I pick the one that makes the most sense for my family and our situation. You need to do the same thing.
If they only offer one plan, there's no deciding. It will almost certainly be a better deal than you can find through an independent agent. This is because the company is getting a group rate from the insurer, and possibly subsidizing it as well. Otherwise you have to weigh the difference in premiums you pay each month/year/whatever against how often you visit the doctor and what you'll pay out-of-pocket each time, and how well you want to be protected against catastrophic illness. Also, if you're still a full-time student and/or under a certain age, you may still be covered by your parent's insurance, so you should check with them. You may not need to get your own insurance at all. My son is 18, but as long as he's a full-time student, he's covered on my policy until he's 25.