Play your part on a piano,while singing along, three or four measures at a time. Mark accidentals (sharps and flats) and pronunciation problems in pencil on the alto part of your score. Highlight the alto part in yellow, but only if you own the score or have permission from the conductor to mark on it.
Play the music on a virtual keyboard using your mouse, if you don't have access to a piano, electronic keyboard, or an iPad. Try "Virtual Keyboard -- Piano," which has a one- and-a-half-octave range. Play the keyboard at "Play the Piano Online -- Virtual Piano/Java Piano," which has a nearly three-octave range, but requires longer to load the Java applet. Try "Interactive Online Piano," which has a two-octave range and works with Macromedia Flash.
Work without a piano, listening to the alto part played on a piano, often with other parts muted in the background, at "CyberBass: Innovative Learning for Choral Singers," which has many well-known choral works. Find your piece, click on "CD," select "Alto", and use the slider bar to stop, start, rewind or advance the recording, skipping to the measures you need to work on.
Use online piano keyboards during your warm-up exercises to keep yourself on pitch and to accompany yourself with chords. This improves and centers your pitch, which is a key part of choir singing.
Find your piece on YouTube and practice singing the alto part along with the recorded music.