Before I get into aiming, I want to iterate the fact that you need solid fundamentals for any aiming system to be effective. It will be difficult to learn how to aim if you don’t have a straight stroke!
The purpose of any aiming system is to give you a systematic way to calculate aim, which you can use as a reference point. From this reference point, you can adjust for sidespin factors such as squirt, swerve, and throw, as well as fine tune your aim.
The physics of billiards is complicated. So complicated, in fact, that no aiming system is perfect — they are all approximations or simplifications of what really is happening on the table. Whatever aiming system you learn, practice it enough to ensure that you can visualize the reference point(s) well.
When you are confident in your aim for the shot based on the system you use, you may choose to use a different spin or speed that will cause the ball’s paths to change due to friction (squirt, swerve, throw.) Fortunately, most pockets are big enough to account for small changes, but you might need to adjust your aim for long shots or for more precise cue ball positioning.
Aiming to pocket the ball is good, but not good enough for high-quality play. For the best cue ball positioning, you need to aim for a particular part of the pocket. The path of the cue ball and speed transfer can change significantly for different parts of the pocket, and many aiming systems do not account for this.
The first aiming system any player should learn is the ghost ball. Visualize the cue ball contacting the object ball, such that the line of centers is pointing towards your target (e.g. the pocket). Your aim point is the center of this “ghost ball,” so the cue ball will replace it. The ghost ball aiming system is easy to learn and provides an excellent reference point.
There are many other more complicated aiming systems that work under various conditions. If you choose to learn one of these other systems, make sure to understand how to adjust, and also realize the limitations of the system.