Heel Brake
Using the heel brake is probably the easiest method for braking. Your braking foot should be stretched in front of the other foot and your legs bent, creating an 'A' shape with your lower legs and the ground. As you grow more confident you can place more and more weight onto the braking foot improving your stop distances.

T-Stop
For this method you use the friction of one skate dragged perpendicularly across the ground behind your direction of travel. Care must be taken with this method at first since it is very easy to spin out of control. You can also apply the heel brake in conjuntion with this method when you become confident enough. One other thing to note is that this braking method is not kind to your wheels and can be expensive in the long run.

Power Stop
This is a very tricky stop to learn but pulling it off in public will increase your street-cred no end. It involves flipping from front to back (if skating forwards) and jamming your strongest skate foot perpendicularly into the ground behind you (i.e. in the direction of travel). It is hard to learn because it is difficult to pull off at low speeds but you can try practicing it on loose/slippery ground without having to go too fast.

Spin Stop
Another very simple stop for the absolute beginner (also with a cool sounding name so you can brag to your mates) is the spin stop which can be developed into the surf spin stop.

You need a wide flat surface for this one....

Glide forwards for a few strokes and then stop pushing and allow yourself to free wheel. Now bend your knees slightly and lean forward. You will now start to turn until you are facing the way you came. As you start to turn your weight will naturally shift onto one foot. You can control the size of turning circle and the direction of turn by putting more or less weight on your leading skate (the one that naturally takes the weight) and by choosing which foot to put weight on.

Try it.....but make sure you lean forward 'cause the centrifugal force tends to make you fall on your back! Ouch!