The quality of any of any decision about your problem is directly related to the quality of the information on which the decision is based. The first step in determining data sources is to determine the data needed. The second step is to address whether primary or secondary data or a combination must be used. Most of you will use secondary data. As you identify your data sources each must be evaluated.
Evaluation consists of three steps.
Data must be tested for validity and reliability and your must review its practicality. Validity means that the data provides objective, accurate information about the research topic. Reliability is a measurement concept that refers to the consistency of results obtained with a measuring device. Practicality refers to the cost, the time, and the convenience of using a data source. There is a lot of data out there, especially on the WWW, but can your afford to use it.