じょうず/へたです. good/bad at doing something

じょうずです

means "be good (at doing something)". Pronouced as "Jyozu-desu". 

ぴあのが　じょうずです

"ぴあの" is "piano". This means "(I'm) good at (playing) piano.". "が" is a particle of object marker. "piano" is a object, so the particle is put after it.

ぴあのが　へたです。

"へた" is the opposite meaning to "じょうず". It means "be bad (at doing something.)". So, this sentence as a whole, it means "(I'm) bad at (playing) piano.".

にほんごが　じょうずです。

It means "(I'm) good at (speaking) Japanese language.". "にほんご" means "Japanese language.".

にほんごが　へたです。

It means "(I'm) bad at (speaking) Japanese language.".