I truly believe that your equipment is largely responsible for how you swing the golf club. If your clubs are not right for you, the result will be a long list of compensations that you have to make in your swing to adjust to the equipment. For years, I had irons that were too long for me, and as a result, I would often sit back on my heels at address. It was the only way for me to make solid contact. I eventually realized that they were too long for me, but this didn't change my bad habit of sitting back on my heels. Despite changing to shorter irons, it took me a few weeks to break this bad habit.
Also, having the wrong equipment can limit your improvement as a golfer. It isn't a fluke that most PGA Tour players have similar styles of equipment. So many golfers wonder why they can’t hit that beautiful draw like Rickie Fowler, but never consider that his equipment plays a huge role in his shot-making ability. Hitting different types of golf shots is much easier if you have equipment that allows you to hit these shots.
Where do I Start??
Since most people do not have the cash or the time to get a fitting and purchase an entire set of golf clubs in one day, I would recommend replacing your bag 1 club at a time. I have arranged this series to reflect this, so please do not feel obligated to read it in order.
That said, if you gave me an old bag of clubs, the first thing I'm going to do is buy myself a driver with a quality golf shaft in it. You can get away with a crooked putter. You can get away with a so-so set of irons. The one thing you cannot get away with is swinging a driver with a flimsy shaft. It will destroy your confidence.
So if you are starting this process from ground zero, I would first focus on getting a quality driver shaft, and then move on to the other parts of the bag.