How to add effect to one layer only

If you are completely new to AE please start with the User Guide or open the Learn section of the Home Screen or the Learn workspace.

This will take you through a guided tour of the UI and the process of adding and animating effects.

If you still cannot figure out what you are doing, select the layer you are having problems with and press the u key twice to show all of the modified properties of the layer. Start changing or resetting things in the timeline so you can see what is going on. If you still cannot figure it out post a screenshot so we can see what you are doing to the layer. No matter how complicated it gets we can see exactly what is going on and could give you some good suggestions. No matter how complicated the project is. For example, this screenshot shows everything that was done to two of the layers in this complex project that I'm working on and points directly to a solution.

Even without asking most of the contributors to this forum would advise me to check the mask on Layer 4.

Hi. There's no reason that this should be happening. Select the layer that shouldn't have the effect and look at its Effect Controls panel (upper left side of your screen, usually paired with your Project panel). There should not be an effect listed in there, if there is delete it.