Locate a keyboard. The vocal range of a tenor stretches from the "C" below "Middle C" to the "C" above "Middle C." Find those notes on the keyboard. If you are not familiar with pianos, find a friend who is musically inclined and they will be able to point it out to you.
Try different vocal exercises to determine if you are a tenor. Before you jump right into singing tenor in a song, you've got to make sure you're not a bass (the other male part.) Play the keys in the vocal range and try and match your voice to the tone. While you probably won't be able to get all of the notes, (especially the high ones) you should be able to hit a majority of them.
Read music to understand how to sing tenor. And if you don't know how, then give yourself a crash course. Notes range from A to G, with sharps and flats in between. They are assigned to the Treble Clef in a specific order. When reading music there are a few tricks. Sheet music consists of 5 lines and 4 spaces. The notes that correspond to the spaces can be remembered as they spell FACE. F is the first space, A the second and so forth. The lines can be remembered by the saying "Every Good Boy Deserves Fudge." The first letter of each word corresponds to the line starting from the bottom. So, the first line is E, the second is G and so on.
Find a group to join and let your vocal chords rip. The advantage to singing 4 part harmony is that there are others who sing your same part. So your little cluster will be able to work and feed off each other as you try and keep the song in tune. Don't be afraid to ask others for help or to "sing in your ear." This is when someone sings in your ear slightly so that you can get the correct pitch of the note when you are starting out.