Gear ratio effect on stopping/skidding

I just bought myself my first fixed gear bike.
And i'm having trouble braking.
I've tried locking my legs but i cant get the wheels to stop turning.
They just force my legs around.
Any ideas why ?

Thanks

You need to lean forward and take the weight off the rear. Even if you are not skidding, you should be able to slow down just by resisting. Also, you should probably get a front brake if you don't have one. No matter what anyone tells you, braking on the front is far more efficient than breaking on the rear. Break on the front until the rear starts to lift, then modulate. Skids are not a very good way to stop in an emergency...