How can I transpose music written for one instrument to the other so they can be played together? ie same pitch
It depends on what sax and what clarinet you actually have. Most clarinets are Bb instruments. That means all the music is written a tone above what you read, so it comes out at concert pitch.
Saxes vary. Mainly because there are several: soprano, alto and tenor are the most used. Alto is an Eb instrument, so the music is written to automatically transpose to concert pitch. Tenor is Bb, as is soprano, the same as your clarinet probably is. Although the tenor plays in a lower range than the clarinet, the music is written in the treble clef, so you will be able to use that music while someone else plays from the same on the other instrument. Playing in unison has a limited life, though - it gets tedious quickly.
If it's an alto you play, then everything will come out a 5th/4th different.
While tenor and soprano saxophones are both in Bb like the clarinet, it is not possible for all clarinet music to be played on the saxophone. The clarinet has a much larger range than the saxophone. The saxophone is limited to just two-and-a-half octaves, while the clarinet can play over three octaves easily (excluding harmonics.) The soprano and tenor saxophones can play in the same key as the clarinet, but what part is given to each needs to be assessed according to range.
The alto saxophone can be transposed up a Perfect 5th from the clarinet music, which is a pretty easy transposition. However, range must be considered here as well. Simply transposing may put a piece out of range for the saxophone, so a key that works for the range of both instruments must be chosen. This may require transposing both parts.
Since the saxophone has the narrower range, I would suggest choosing a piece and a key that works well for the saxophone and making the clarinet part work around that.