That looks pretty rusty. If you don't have a water heater pan, you can try putting the grate in a large garbage bag, then soak it really well with an oven cleaner spray....close the bag, let it sit there for a day or so, then hose it off. Hit it really good with a wire brush. Let it dry, then spray with a cooking oil like Pam. See how it looks after that.
If it's really pitted and far gone, you might need to just buy a new one. The Weber one's are affordable. KillaGrilla makes some very heavy duty stainless steel grates that will probably last a lifetime, but are little pricey. Free shipping if you spend over $200.
Like Timothy mentioned, after you get them cleaned, take the center piece out if your using the charcoal baskets in the center or if using a Vortex. Both will burn up that center piece pretty quick.