This really needs a proper chat with us, but here's a few pointers to give you an idea.
There are numerous shapes and sizes out there, so knowing what style and construction to choose is a very difficult task. Paddle style and length all depend on your boat choice: What's appropriate for a plastic touring kayak isn't going to be the same for paddling a lightweight skinny racing k1 or k2.
Depending on budget - prices start from as little as £40 for a heavyweight plastic bladed paddle. As the budget increases, the paddles get lighter and lighter - going up to £400 if you want to - But ensuring you have the right length and blade size relative to your craft and your physical build is crucial - for that we'd recommend talking to an expert.
There's also the choice in racing boats to choose between assymentric (flat) blades and Wings (spoons!) Paddle on both and you'll soon realise the wings have far more purchase in the water, and feel far more efficient. If you have perfect technique, and can keep your style nice and high throughout the race - the wings can give you a 15% advantage - however the reality is that comes at a cost.... They are less efficient at support strokes, draw strokes and steering strokes - they are best used in shorter distance racing, as as soon as your hands drop down when you start to tire or concentration wavers (sometimes within a couple of hours) all advantage is lost. If you over rotate with a low style, you'll also pick up water on each blade, and end up weight lifting the length of the race!
If you plan on using wings, we'd recommend getting some coaching - talk to us, or get down to your local racing club and get some pointers.
There's even the choice of a one piece or split two piece paddle. A one piece has nothing to go wrong, but a two part is easier to transport, and some paddles can even vary in length and feather too.
If you're buying paddles for a junior, we'd recommend getting adjustable paddles, that can change in length as your young athlete grows.
Finally - particularly in relation to wings, don't buy anything without trying them first - Different blade sizes, styles and shapes all feel different, so you need to find one that matches your style and physiology as much as it matches your budget.