Welcome to the wonderful world of the cello! It`s a lovely instrument with a rich resonance. I too came to the cello as an adult after studying piano for many years.
In answer to some of your questions: The cello has 4 strings ADGC, "A" being the highest pitched. It`s a key of C instrument, but of course you can play pieces in any key. It uses mostly the bass clef, but sometimes in order to avoid using an excess of ledger lines the music will be written into the tenor clef or sometimes the treble clef. (The tenor clef uses the moveable C clef sign, placed on the 4th line locating middle C there.)
You should be able to find out lots more info on the following websites:
http://www.cello.org/heaven/
http://www.cello.org/
Hope this helps,
Donna N.