@yukinoh1989
You can add modifiers with the blue key on the proprieties pannel
The simple deform modifier is in the “Deform” categories
Taper is an option when you add a simple deform modifier to an object 🙂

Here is the documentation about the simple deform modifier if you want to know a bit more:
https://docs.blender.org/manual/en/dev/modeling/modifiers/deform/simple_deform.html
😉