The most common problem I see in golf swings is Coming-over-the-Top or having too steep of a plane on the downswing. It often leads to pulled shots, slices, casting the club and often produces divots that start behind the ball. Golfers usually try to fix the problem by "swinging to right field", "hitting the inside portion of the ball", or aiming well to the right of their intended target. But if you really want to get on the correct plane approaching the ball (slightly inside on the arc), you must correct the cause of the problem, which 9 times out of 10 is The Takeaway.
Lets first talk about a One-Piece takeaway and the misconception behind that term. When you look at a good swing, it will appear as though a nice triangle between the shoulders, arms, and club stays in-tact during the takeaway.

But what is actually happening is the sequence of the swing has begun with the club moving first followed by the hands, arms and then the shoulders. The feel of the takeaway varies from person to person but you should probably notice a few of the following sensations:
1-Clubhead swinging
2-shoulders rotating in a rocking motion
3-arms swinging straight back away from the target
A few sensations not to feel:
1-Independent hand movement or wrist break
2-Arms rolling or turning
3-early hip turn
Checkpoints to practice:

- Clubhead stays outside of the hands, left upper arm stays close to your chest

- Left shoulder moves down and back under your chin
If you practice this takeaway, you should start to notice that your over-the-top downswing starts to go away over time.