Yoga is a good place to start looking as it can start quite gently. 'The Cat' I think you call it, is one I remember (but there are likely to be man, depending on the particular difficulty).
Basically you start on all fours and, whilst breathing out, arch your back and push your head down towards your groin and you groin towards your head. When you breath in you roll you head back, pushing out your chest and lift your lower spine/hips to curl your 'tail' upwards. Do thi slowly and deliberately. Better still take up yoga!