Useing a flash off a sony a300
Obviously, you'd need a separate flash unit, since you can't take the built-in flash off the camera. Just as obviously, you'd need a way to trigger that flash.
Some flash units have a remote sensor that will respond to the camera's flash. The a300 has a wireless mode for triggering such flashes. There are remote flash systems that include a radio transmitter that fits onto the camera's hot shoe and a receiver that the flash mounts onto. Some flash units have a jack so you can use a cord to connect the flash to the camera.
The degree of automation depends on the system. The a300's wireless mode works best with Sony's own flash units. The separate wireless and cord methods will work with just about any flash, though you'll probably have to set the exposure and flash output yourself.