You will want a graphics tablet, but there are many to choose from. Some things to consider:
Is art your hobby or your job? If you are a professional illustrator, for example, you will need a larger tablet, but that will cost more and you should also consider whether you are on the move a lot - a large tablet may not be practical. Check the footprint (ie the total size when it's actually sitting on your desktop) rather than the screen size.
What sort of computer do you have? An older one may not be compatible with a tablet, so do check this.
Do you want a tethered or a battery pen/stylus? A battery means you don't have to attach it but will be heavier and needs charging. A pen with an eraser on it is very useful for artists.
You should also look at the pressure sensitivity on your tablet surface - the more sensitive it is, the better for your drawing.