If you have a personal tutor this is a good question to ask him or her early on.
Many universities don't monitor lecture attendance, in which case no-one will notice that you aren't there. Some do monitor attendance, in which case there might be a form to fill in or person you should e-mail.
It's polite to e-mail the lecturer (although she probably won't know whether you're there or not if it's a large class) to let them know. Ask if there is anything you should do to catch up. Good idea to ask a friend to share notes with you afterwards.
There may well be a policy document buried somewhere in the departmental/university website for these situations but good luck trying to find it! Best thing is to ask a personal tutor or course leader or relevant lecturer as soon as you can.