19hole has it right.
Start by seeing a pro. Even if you only see them once or twice, take to heart what they tell you and show you. Most people who hang around 100 have some very fundamental issues (posture, grip) that can be worked on with a teacher. Once your fundamentals are right, they can work on swing path, ball positioning. They also help determine if your clubs are appropriate. Yours might be too short, not have enough loft and various other issues.
Once you have those issues sorted it becomes mental. Playing smartly within your limits. Do you hit a fade every time and end up in the woods? Well stop aiming down the middle and aim left. I found for me, once I stopped trying to hit the ball a mile I gained a TON of control. I could still poke it out there, and the 10 yards less I was hitting it was actually beneficial because I could find the ball!
Lastly, practice, as much as you can. Practice your fundamentals. You dont need to actually hit a ball, use a full length mirror and work on your posture, your grip, your swing plane. And then get out there and work on your short game and putting.
There are lots of avenues to improving your score. You just need to figure what exactly is wrong first.