In the getParameterInfo() method, you'll define your parameters as arcpy.Parameter() objects. For the respective parameter of your ValueTable ensure that you are using the 'parameterType=GPValueTable' constructor argument.
Once you have created the Parameter object with the correct type, you can use the ValueTable's methods such as setColumns(), setRow() and setValue() to alter the values therein.
See this link for info on creating ValueTable Parameters and this link for the ValueTable documentation
