First find a bird that’s not flying. For example, focus on one that’s on a branch, but don’t zoom in too much — you don’t want to frame the bird very tightly.
A lot of birds crouch down really quickly the moment before they fly off. So keep your finger half way down on the shutter button, maintaining focus and ready to shoot, and the instant you see the bird crouch down take your shot. It’s going to happen really quickly. Fire the shot the instant you see it crouching down. By the time your camera’s responded the bird will already be flying and with a bit of luck it will still be in focus and still in the frame.
So that’s one trick for photographing birds. There are more tricks over here.