Here
http://en.wikipedia.org/wiki/Counter the workings of counters are explained.There is even a figure using JK-flipflops - just what you need.
Without further circuitry, a binary counter will count powers of 2 (2, 4, 8, 16, ...).
Since you want to count only from 0 to 14, you will have to devise a circuit that will make the counter go from 14->0 (instead of 14->15), You need to detect the counter state 14 and will have to use this detector signal to control the JK-inputs of the flipflops such that on the next active clock edge the flipflops all will reset to 0.
Hint: An obvious solution would be to detect the counter state 15 and use this signal to reset all flipflops asynchronously. However, this approach has several drawbacks and is therefore not recommended:
1) the state 15 shows for a short period of time at the outputs of the counter. This may confuse other logic.
2) If the flipflops behave drastically different, not all flipflops may be reset (assume one especially fast flipflop going to 0, then the state is no longer 15, the reset signal is cleared and some especially slow flipflops may not be reset in that time).
3) Mixing asynchronous and synchronous operation may lead to so called hazards or races, both conditions that can wreack havoc in the following logic stages.
Harald