If you can do something easily are said to be?

* Adept

* Skilled

* Proficient

* Competent

* Talented

* Gifted

* Able

* Capable

* Dexterous (if it's a physical task)

* Savvy (if it's a mental task)

* Fluent (if it's a language or communication skill)

* Natural

* At ease

* A breeze (informal)

The specific word you choose will depend on the context and the level of ease you are trying to convey.