Hey Dottie,
When using an Electronic Tuner, (There is a ton of them) usually you use your finger, or thumb. After plucking the string, let it settle down a bit. Some have a needle that center's when the string is in tune and uses colors to indicate that the string is flat or sharp. Sometimes the Electronic Tuner will go haywire, so to speak and register another string. After getting the guitar in tune with the tuner check the strings against each other. Then strum the guitar to see if it sounds right. It takes practice but you will get it in a short time.

Ray E. Strode