The most common mistake I see newbies make at the helm is that they oversteer the boat. They steer it one way, and then, when it's going too far in that direction, they steer it the other way. As a result, the boat oscillates back and forth, instead of holding a reasonably straight course.
Let me suggest an experiment that will help you understand an important concept about good helmsmanship. Ask a good helmsman to put the boat on a course, and then lock the helm, so that it can't move. You'll see that the boat will hold it's course for a short while, even without anyone touching the helm. What that tells you is that you don't have to constantly make big adjustments in the helm. Let the boat more or less steer itself, and just make occasional adjustments when necessary to keep it on course.
It's really the same principle as driving your car. When we drive a car, we don't constantly make big adjustments to the steering wheel. If we did, we'd zig zag all over the road. We learn with experience that the car will almost steer itself, and all we have to do is apply very slight pressure to the steering wheel, one way or the other, to keep it on course.
Secondly, when the boat strays off its course, don't bring it back on course with one big adjustment of the helm. Instead, turn the helm in the direction that you need to go, and hold it there until the boat is almost back on course, but not quite. Then ease the pressure on the helm slightly and let the boat settle itself onto that course. When it has done so, then make another small adjustment of the helm to bring it the rest of the way onto the desired course.
Finally, the only way you'll learn to steer the boat is if you spend some time with your hands on the tiller or wheel, practicing. Take your turn steering the boat whenever you can. It won't take long for you to get the hang of it.