The information can be found in Act II scene 1, which is when Iago has a chance to observe Cassio and Desdemona together. What he does is to persuade Roderigo, who himself sought the hand of Desdemona in marriage, that Desdemona is in love with Cassio:
lieutenant tonight watches on the court of
guard:--first, I must tell thee this--Desdemona is
directly in love with him.
Of course, one of the important aspects of Iago's character is the way that he is able to read those around him so excellently; he understands that Roderigo will be driven into anger by the thought of Desdemona loving anybody else but him, and thus he uses this fact to get Roderigo on his side. Iago really shows himself to be an expert reader of people, as he knows just how to manipulate Roderigo and what would work best to enlist his help.