This is a smart question and there are factors to consider when deciding what pump to buy.
First and foremost: find a licensed, certified pump installer and well driller. He can be of great help describing the options available based on your particular needs.
Second: whether replacing a pump on an existing well or installing a new well, determine depth of well, standing water level, gallons of water per minute needed and what size pump is already in place. Most likely, the original pump on an existing well is ideally selected for the situation at hand.
An easy formula to determine just how much water is needed follow this:
Every faucet in the house should be considered even though not all will be running at the same time. Each one requires one gallon per minute. A typical bathroom will have three or four (one or two sinks, toilet and bath/shower), a kitchen might have a sink and a dishwasher, laundry room a single supply plus any outdoor faucets. So if there are 10 fixtures requiring water the pump should supply at least 10 gallons per minute.
A one-half to three-quarter horsepower pump should be sufficient in most cases, unless the well is deeper than normal.
Again, a licensed, certified pump installer and well driller has the knowledge and experience that can be of invaluable help.