Focus on a fast arm swing
Many players fail to get the most out of their hits because they think “hit hard” rather than “swing fast”. Power in a spike comes from the speed of the hand at contact. A good arm swing starts with an open torso (which should come from proper approach mechanics) as power begins with twisting through the core with the elbow drawn back. That then carries through the shoulder as the elbow comes forward. It finishes with the arm extending at speed to ball contact. Players get into trouble when they swing from the shoulder, not just in terms of inefficient spikes, but also in increased likelihood of injury.
Contact at full extension
Execution of a proper fast arm swing as mentioned above will see the player strike the ball at the highest possible contact point. This is critical in many ways. Obviously, it creates the best possible attack angle. Players, however, will often drop their arm. Not only is this less efficient, it also leads to a lot of hitting errors. This is particularly noticeable when hitters are trying to hit down the line and when they have no block.
If you can get your hitting mechanics working properly, that alone will make you a much better hitter. From there you can then work on being able to mix things up. Good hitters have the ability to change speeds and to hit to all areas of the court, especially in terms of being able to show a cross-court hit and do a line shot or vice versa. Those skills all require a strong mechanical foundation, however, so start with that and you’ll find yourself progressing nicely.