Lock. The 'o' is pronounced like the 'o' in 'hot' or 'pot'.
Luck. The 'u' is pronounced like the 'u' in 'cup' or 'mug'.
If you know the phonemic chart, the 'o' sound is /ɒ/ and the 'u' sound is /ʌ/. There are some very good websites on the phonemic chart to help with pronunciation.