For a source to be credible it has to be written by an authority on the topic. Everybody can express an opinion or have published something about a specific subject, but for you to use it as an important argument it has to be made by someone or something, in the case of an organization, that is an authority on the topic and has studied the subject properly.
The starting point for research is to identify the research question. You have to have a very clear idea of what is it that you are looking for so that you can have an idea on how to find it, and which are the boundaries you don't need to cross because it is not essential and it would be a lost of time.
Boolean logic is a method for searching the Internet. When an "and" operator is between two words or other values it means one is searching for documents containing both of the words or values. When an "or"operator is used, it means one is searching for documents containing either of the words.