Be careful which aquatics store you chose though - some give abysmal advice and seem to just sell whatever they feel like at the time.
You'll need to know the rough volume of your pond (from it's dimensions) to help you get the right system. You'll also need to know the maximum head required i.e. the distance from the pump to the top of the waterfall if you have one. If you do have a waterfall and it is a big one you may find it more economical to run two pumps - one for the waterfall and one for the filtration system.
If you have fish in the pond you need a filter really and that will need to be driven by a pump. The majority of biological pond filters require a pump that is capable of circulating the pond water every one to two hours. So if your pond has a volume of 1000 litres then you need a pump with a flow rate of between 500-1000lph.
If your pond is stocked with koi you might want to circulate the water a bit more than that.
Choosing your filter isn't all that hard once you know what flow rate you want - they all give flow rates on the boxes. UV lights encorporated into systems are useful to keep algae in control but check out the price of replacement bulbs before you buy.
here is an online flow rate calculator.
this website is also very useful for people new to pond keeping. This link takes you to the pump flow rate calculations and goodness knows what else as well!
Alternatively - ask the seller while you can!
btw - this all looks a lot more complicated than it really is!