Recording how you know something to be true is one of the most important practices in genealogy. Many genealogists regret not doing so until it is too late.
Sources in Pedigree Forge are entered and edited via the Sources panel, accessed from the second button on the Action Bar. The panel shows a table of all sources in the project. Clicking a source makes it active for editing; the buttons at the top let you add, delete, or reorder sources.