How do you find the correct charger to buy for a laptop?

For the Lenovo Essential B570 there should be a sticker on the underside of the laptop. This sticker provides all of the details needed for what charger is required.

If your sticker is missing, the Input voltage should be 20Volts and 3.25 Amps. When deciding a charger, you can see the volts and amps they provided listed on the chargers description online, or on the actual charger itself.

Also, make sure that the charger actually fits in the port. When buying the charger from the manufacture (Lenovo in this case), check to see the listed laptops the charger will be compatible with.