The only instrument you could have trouble with in the string orchestra would be the viola, then. Just think of it this way - if you are familiar with the grand staff, just remember that that alto clef sits right in between the treble and bass. The middle line of alto clef marks your "middle C". You can get that in your head and compare all the other notes to that note by intervallic relationships until you're fluent in recognising which lines/spaces are which notes.
Also, if you get really stumped, try this. Think of the notes in other clefs that would be the note you are looking at in alto clef. If your note looks like a D in treble clef, but it also looks like an F in bass clef, it's the note E in alto clef, which is between D and F. This only works for alto clef, none of the other C clefs.
If you're writing high 'cello music and feel the need to use tenor clef (a lot of inexperienced AND experienced 'cellists hate it, by the way... ), tenor clef is the same as alto clef, only it's shifted a line. You'll notice the center of the symbol is on a different line, so now THAT line is middle C. In this case, middle C looks like a D in treble clef.
Here's a little quiz that helps you learn alto:
http://courses.wcupa.edu/frichmon/usetech/musicalflashcards/acnotes.html
Have fun, and good luck! Just remember, practice lots and you'll learn it in no time.
