Well, they're pretty much like regular bananas. When they've gone all yellow is a good time. The riper they are, the sweeter they'll be so feel free to experiment and try some out a little earlier than others. People like different tastes.
Green plantains can be treated something like vegetables. Fry 'em, mash 'em, sautee 'em or add them to random dishes.
If you want to try green plantains, maybe you can make tostones? Soak the plantains a bit and fry 'em up till they're a little soft, then smash them a little and refry till crispy. And then you can season them with some salt, or garlic salt or what have you.
Riper plantains can be fried up as a dessert. With some yogurt or pudding, they can be very good.