How can I improve my strength for Golf?

Most of the power in golf is from a combination of legs, core, shoulders and arms. On the one hand, you have to ensure that you get a balanced workout and strengthen all major muscle groups, as this will ensure that you don't have strength imbalances between muscle groups that can lead to injury and possibly lack of coordination.

The core is the key and I would focus on doing medicine ball throws from lying on a Swiss ball and other ballistic abdominal movements and including the obliques by some kind of "axe chopping"-type exercise (for lack of a better description).

Also, do deadlifts and power cleans - these will really allow you to develop power in your swing. Other exercises to focus on are calf raises, shoulder flies (especially reverse flies for the rear deltoids) and biceps/triceps movements.

I was told that stretching for golfers is really important.

I'm a golfer as well. I've improved my drive over the last few years to average about 270. In this time I've focused on regular large muscle group training and yoga. The flexibility with yoga gives you a larger swing path. More range of motion increases club head speed which make the ball fly off the tee. I would recommend yoga more frequently, but don't neglect the stregth training.