Find a piano. The best way to determine if you can sing a song in the alto range is to pluck a few notes. Altos generally sing from the "G" located below "Middle C" to the "E" a 1/10 above "Middle C." To locate "Middle C" look for the key directly below the name of the piano vendor above the keyboard. Traditionally it is located there.
Perform a vocal test. Don't fret, you don't have to suddenly belt out a big Broadway song. Simply play the keys from "G" below "Middle C" to the "E" in the following octave. Now try and match your voice to each note. If you are meant to sing alto, then most of the notes you play should comfortably match your voice. It is normal to strain at either end of the range.
Educate yourself on how to read music. It may seem a daunting thought at first, but a little education will do wonders when you sing in your choir or vocal group. A quick study of the treble clef will allow you to assign names to the notes you are reading. (These are the letter symbols previously discussed.) Once you begin to recognize these notes with the letters they represent, you will be well on your way to singing alto.
Join a vocal group and sing alto. You have the tools, now have some fun. Locate a group with your school, church, or community that sings 4 part harmony. Once part of the group you will be placed with other Altos who are singing the same notes and parts as you. This is immensely helpful as they can sing "in your ear" as you are learning and getting your sea legs.