Here are a few questions to ask yourself.
If you answer "yes" to any of these questions, then you need to take a lesson.
First, a slice is a weak shot. It does not go far. It is not solid. And it does not hold up in the wind. The only time that you should play a slice is when you can already play a hook or draw. After all, the best players in the world have fought a hook at some time in their career and you don't.
STRUGGLING IN ONE AREA
If you struggle with a certain aspect of the game, you need to turn your weakness into your strength. That way all aspects of your game can be well rounded by improving in all areas. It doesn't make sense to practice what it is that you are already best at. Often times people are better at one area and lesser at another area but don't know why they struggle.
If you want to lower your scores, seek qualified advice. Don't listen to someone that can't beat you. Often I see friends helping each other out with their swing, which is great. But I never have understood why a person wouldn't seek the best advice possible. What I usually hear is one person telling the other about what he is working on in his own swing or what he doesn't like about the other's swing or something that is totally wrong for that person's mistake - never anything that would have an immediate impact and improvement on the other person's game.
Golf is a game designed for your enjoyment but most people only become mad and enraged after a casual round. Taking a lesson from a qualified teacher does a few things for you. It allows you to have another pair of eyes to help dissect your swing. It allows you to have your questions answered by a professional. Most importantly it gives you something to work on when you practice, so that you are practicing with a purpose, and not just out exercising or trying new things that will only make you worse.
Most people have had bad experiences with a lesson somewhere in their history. They may have been told that they had to get worse before they could get better. This simply is not true. That is only a cop-out when the instructor is out of guesses. When I go to the doctor I don't want him to say that I have to get worse before I get better. I want to improve now. The best players in the world get help with their swings, so should you.
FINDING A TEACHER
So how do you go about finding a good teacher? Ask around. Look for someone who teaches a lot, not the guy behind the counter working part-time. Talk to the instructor and find out what his theories are. Watch him or her teach a lesson. Look at their attitude, personality, and demeanor. Talk to their students. And do not be afraid to ask questions during your lesson. If they can't tell you the reasons why you are about to make the changes you are going to make, then they are only guessing and hoping for results. After all, you want to trust what it is they are telling you. You have to have a plan. What is it you are trying to fix; your swing or your ball-flight? Don't make changes just to look better. Make your weaknesses your strengths. Stay committed and stay patient. Give the instructor a chance. Just as you can only be as good as the information you are getting, your instructor is only as good as what you pour in to your practice and how well you stick to your plan. Schedule a follow-up lesson so that you can have a check-up. This is important because it allows you to know if you need to continue to work on the correction or if you are over doing it. To change your scores you have to change something that you are doing. A good instructor can help you make the correct changes.
Impact causes or determines your shot pattern. The swing leads to impact. Your hands, arms, shoulders, etc. affect the swing plane, which is the golf swing. What typically happens is that you are trying to get the club to do something different to affect your impact and thus hit better shots.
What happens after you've taken a lesson? Any changes that you have made should have been to change your ball flight. Now you have a plan, something to work on and practice. Spend time implementing that plan on the range, not on the course at first. Give yourself a chance to see some change in your ball flight. That is why you took a lesson in the first place.
Those changes probably feel weird or awkward. Remember that you are not trying to feel comfortable. You are trying to feel correct. Repetition will have the change feeling comfortable soon, so stay patient. After the change is starting to feel better and your shots are flying straighter, it's a good time to go and play. There are only three ways you could respond to your change: over doing the change, the correct amount, or not doing it enough. If it is correct, then you can work on other areas of your game. If your shots start going back to the old patterns, then you need to exaggerate your correction until the ball flight gets back to where you want it. Your swing is like a car; a little scheduled maintenance will keep you in top shape, so schedule a follow-up lesson.