Do I have to say watashi wa every time?

After I say “Watashi wa” the first time, can I just omit after that in my sentences?
For example,
“Watashi wa nihongo o benkyo shitemasu”
Can I just say “nihongo o benkyo shitemasu” instead?

And
“Watashi no nihongo wa hetadesu”
Can I say “nihongo wa hetadesu” ?
Or do I still need to say watashi no.

Also do I have to say desu after every sentence? Thank you