watashi wa

onamae wa yuki yamada desu.

namae wa yuki yamada desu.

Honorific word should be used to others, not to yourself.

watashi wa 21 sai desu.

watashi wa echiopiajin desu .

watashi wa igaku no gakusei desu.

These three sentences are very good, however, omitteing 'watashi wa' from second and third ones to make your Japanese more natural.

nihon no bunka ga omoshiroi desu kara nihongo benkyoushite imasu.

Good, perfect:)

 
nihon ni igaku wo benkyoushitai desu.

If you would like to study it AT Japan, you can say that 'nihon DE igaku wo...'

But studying Japanese medical field is all about you want to do, the sentence comes to be like this

'nihon NO igakuwo...' . Particle 'NO' is to modify the noun while particle 'DE' is to point the place where you are doing sth at.

Entirely, the sentences you put down here is very good, nice to be read:) Hope seeing your posting again as soon!

Murooka