I'm good at speaking Japanese.

Watashi wa Nihongo ga tokuidesu. : I'm good at speaking Japanese.
Nihongo : Japanese(language)
tokui : be good at
...ga tokui : be good at...
Watashi wa Nihongo ga tokui desu.: I, Japanese, good at : I'm good at Japanese.

futokui : be not good at,be bad at
Watashi wa Nihongo ga futokuidesu. : I'm bad at Japanese.

Watashi wa kuruma no unten ga tokuidesu. : I'm good at driving a car.
kuruma : car
unten : driving
kuruma no unten : driving a car
Watashi wa kuruma no unten ga tokui desu. : I, car, driving, good at. : I'm good at driving a car.

Watashi wa tenisu ga tokuidesu. : I'm good at playing tennis.
tenisu : tennis
Watashi wa tenisu ga tokuidesu : I, tennis, good at : I'm good at playing tennis.

Watashi wa Nihongo ga amari tokui dewa arimasen.: I'm not good at Japanese very much.
amari ...arimasen / nai : not...very much
tokui dewa arimasen : not good at
Watashi wa Nihongo ga amari tokui dewa arimasen. : I'm not good at Japanese very much.