Diagnosis
Unfortunately, it's a frequent complaint. The steel used for the firebox and the grease management system isn't stainless, so it's prone to surface rust, especially if moisture and grease sit in the tray.
How to Fix It
For the grease tray, empty it after every few uses. For existing surface rust, pull the tray out, scrub it with a wire brush to remove loose rust, and coat it lightly with a high-heat cooking oil (like canola) to create a barrier. For the firebox interior, after a hot grill session, use a stiff brush to knock off any flaking material. There's not much you can permanently do for the exterior paint, but keeping a cover on it is key.
If That Didn't Work
If the rust is perforating the metal (creating holes), the parts are failing. Char-Broil sells replacement grease trays and flavorizer bars. The main firebox is harder to replace and might mean considering a new grill if it's structurally compromised.
Heads Up
The single best thing you can do is use a fitted, waterproof grill cover and keep the grill in a dry location. Letting rain pool in the grease tray is a surefire way to accelerate rust.