Optimal Control of State Space model, Where do i start?

i have a 2input 2output cross coupled state space model. i need to use optimal control to achive the best results for decoupling ability. Knowing almost nothing about this method (mainly just a little about varing the weighting of variable in th performance index can achive different results) could somebody give me some idea of where to begin, what simulink model blocks do i need to use? toolboxes? all i have at the moment is a SS model and i have no idea what to do.

thaks in advance.