I'll explain just some basics of what to look for.
First, the price of a drone or quadcopter affects the quality of it's camera. To film in High Definition a camera should be one or more megapixels. That is HD, but a lower quality of HD. Usually with the resolution of 720p. Filming with a camera of two or more megapixels will bump up to 1080 FullHD.
Second, is the stability of the camera. More drones are equipped with a stabilizer. This softens the amount of vibration from the movement in flight and the spinning from the props. This helps for better footage but in most cases it just dampens the vibration . Remember that not all stabilizers are created the same.
With that said, lets move toward technology's help. This opens up the motorized axis gimbal. You have mini servos that keep the camera very stable. Leaving your video footage ultimately seamless with no jellowing or wobble. This gimbal becomes your best friend.
Here is a good example of bad footage below.
I know this is a relatively new venture for you. Just keep on polishing those skills and your patients will always payoff. Fly safe, drones don't hurt people. People who don't take the time to know their machine hurt people.