Using front and rear brake
You have to overcome the car-derived impulse to mash the brake pedal with all your might in a panic situation. That's going to mean a rear-wheel skid, and then a highside when you release the back brake.
Learning to ride well is all about suppressing your survival instincts and replacing them with new ones. One of them is to not tense up and clamp down on everything in an emergency situation. Smooth, progressive braking will save your ass most of the time.
Your front brake gives you 80% (or more) of your stopping power. As you brake harder, weight transfers to the front of the bike, compressing the forks and taking weight off the back wheel, giving the rear tire even less traction. That's why it's easy to lock up the back.
So, when you are learning braking, focus on using the front until just before skidding the front. Do this in a warm, empty parking lot free of gravel or sand. Practice stopping from 30 or 40 MPH until you can stop as quickly as possible. Once you're comfortable with the front, add in practice with the rear brake.
One thing to remember is not to forget to use the rear brake. Get used to how it reacts in a panic stop. That will give you more stopping power than the front alone. Also, the rear comes in very handy at slow speeds. Using the front while turning at parking lot speeds may cause you to dump the bike. Along these same lines, dragging the rear brake, in conjunction with applying the throttle, can help you make u-turns more easily. Try it and practice it. The rear also plays a big part in trail braking.
And don't worry about those big boys you see out there with twin rotors up front. Two front discs on a 250 would be overkill. You can lift your back wheel in a panic stop, even with a rubber brake line and a warped disc. Two discs would leave you wearing your bike as a hat!