I meant an assignment for school, in which case it's best you do it yourself with the tools you've been taught to use to-date. Asking others for help on school assignments not only hurts your learning process, but is often obvious cheating because tools you haven't been taught to use are in the circuit.
But that doesn't apply to a hobbyist such as yourself, so here is my advice:
The device you need is called a
Flip-Flop. There are a few kinds, you should use a T-flop or a JK-flop. Using just one input, you toggle between high and low. It has a Q and a notQ output, one for each LED. When one is high, the other is low.