I am starting to study the clarinet and have been given some sax music. As both instruments are in B-flat, if I use the clarinet to play the sax music: what must I be aware of? are there octaves differences etc.?
If the sax music you have is written for an instrument in B-flat (tenor or soprano) then you can play it on a B-flat clarinet with no problems. Note, if the music is intended for tenor sax then it's going to sound an octave higher on clarinet. You might want to play some of it an octave lower where it's possible and you think it's musically appropriate.
If you had music written for a sax in E-flat (alto or baritone) then you would need to transpose it down a perfect fifth.
It is true that the standard clarinet in B flat can cover a lot of tenor pieces. They are both written in B-flat, which means you won't have to transpose your sheet music. However, there are a few things to keep in mind.
The sounding range differs by an octave. When you read the same note on a B-flat clarinet written for tenor sax, you'll play a note that is an octave higher. Up to you if you want to play the notes as they are written (but an octave higher) or as they sound (so you play every note an octave lower than it is written).
The ranges differ. A tenor sax plays from a sounding Ab2 to roughly E5 (you can go higher with overtones) whereas a B-flat clarinet without extra low keys can only reach a concert pitch D3, a tritone higher. It is able to play higher though. So if your tenor piece features notes lower than a written E above central C, you won't be able to play it on the clarinet at the same pitch as the tenor sax, but you can still play the whole piece an octave higher.
The register keys differ. A tenor sax has a register key that transposes by an octave. This puts the 'break' between written C#5 and D5, whereas in the clarinet, the register transposes by a twelve, and has the break at a written Bb4 to B4. Pieces written for clarinet normally take crossing the break in mind, but there's no guarantee in a tenor sax piece. This means that you must check if your tenor piece doesn't cross the break in quick scales (unless you're very proficient of course).
If you check the score using these tips you can check if it's possible for you to play the piece. Good luck!