Credible, reliable, trustworthy, authoritative - these are all words to describe sources that you can trust to contain accurate information. All your sources for any speech should be credible.
One method you can use to evaluate the credibility of a given source is the SIFT method. This method consists of four moves that help you look at the source in the larger information context around your topic. It is designed to be completed quickly so you can determine which sources aren't appropriate and which ones are worth more in-depth consideration.
Relevancy describes how useful a resource will be for meeting your needs. Sometimes a very credible or reliable source can turn out to be not relevant for your topic. Here are a few things to look at when determining whether a source is relevant or not.