Here's what to do:
Click on the Modules tab and scroll down to the Customer loyalty and rewards scheme. Install this module.
Click on Configure. You will see something similar to the following screenshot:
Most of the fields can be left as they are, with their default values. Here is an explanation of the interesting or important ones. First, in the Ratio field, enter the number of US dollars ($), British pounds sterling (£), or Euros that a customer must spend to earn a point.
Underneath this field, you will have to enter the value of a single point as the decimal fraction of your chosen unit of currency. Do the math carefully for this value. For example, if you leave in the default values of $10 per point and the value at 0.2 per point, then for every $10 your customer spends, they will get 20 cents. If this is not generous enough, you can reduce the required spending per point, say, to 5. Then they would get 20 cents for every $5 spent. Or...