You will always be playing in a key that has 3 more sharps or three fewer flats than the piano. If the piano has one flat, the Eb saxophone will be playing in 2 sharps. You will always play (on alto) one sixth higher--easier to read one third lower, and then raise it an octave. It is a minor third, but if you've alread figured out the key, that's taken care of. Also, remember that written accidentals will also need to be adjusted. For example, a B that is flat in the key signature which has a natural accidental, will need to be sharped as it is transposed, a G#. If you're reading of the bass clef (as a bari sax might do), it's much easier. Figure out the key, then pretend it's treble clef (a written C in bass cleff looks like an A in treble clef). The Bari Sax in our high school sometimes played off the tuba music by doing that...

When you practice your scales, use Solfege (do-re-mi-fa-so-la-ti-do) to relate to the function of the notes (moveable "do" with a "la"-based minor). Sing the Solfege syllables in your head while practicing. Get used to sightsinging the music you read in Solfege. When you need to transpose, you can "tell" your fingers to play in a certain key if you have developed a sense of function. This takes a lot of work, but if you practice it properly, you will be able to transpose to any key.

Pretty basic. Just read a major 6th up (or a minor third down) and add three sharps to the signature. Example:Piano in C Major, Eb alto in A major (3 sharps) or piano in F major (1 flat), Eb alto in in D major (2 sharps). Remember that if the piano has flats in the key signature, each flat will cancel a sharp in the key the alto sax music.