If you are just starting out in strength training , twice a week is perfect. You need to give your body enough time to recover and benefit from the training and 20 – 30 minutes is plenty. Generally, you would start out with between 6 – 10 exercises at a low weight or light bodyweight resistance with a mixed routine of upper and lower body exercises as well as possibly 1 or 2 core or balance exercises. Focus on higher reps and lighter weights.
In terms of which exercises, starting with simple bodyweight exercises with good technique is the key. Squat, lunge, half/full push up, side lunge, plank/half plank, curls, presses and dips would be examples of simple but effective exercises for building basic strength.