Welcome to the forum! Have a look around, lots of great information.
A good diet consists of 10% seeds, and pellets should be the base of the diet, making up 30-50% of the diet, and the rest can be vegetables. Make sure you feed plenty of fresh vegetables, and you need to check to see if certain vegetables are toxic. Supplement the diet with sprouted seeds too, like alfalfa sprouts for example. There are lots of foods you can feed as treats too, I make scrambled eggs for my cockatiels in the morning.
I hope that this is of some help to you.