First of all, Hhi!

I'm Yaren and I'm 20 years old.

I'm studying at prep school.

I hope to improve my language skills and I hope this app will be good for me.

First of all, Hhi!

Lowercase because it's not the start of a sentence

I'm Yaren, and I'm 20 years old.

I'm studying at a prep school.

"Study at [place]" and should have an article

I hope to improve my language skills, and I hope this appwebsite will be good for me.

Two verbs next to each other require "to" between them (unless it's a modal verb)
Apps are downloaded, websites are just any sites on the internet you can go on

First of all, Hihello!

The formal beginning "First of all" sounds better with the more formal greeting "hello". Only capitalize the first letter of a sentence.

I'm Yaren and I'm 20 years old.

I'm studying at prep school.

I hope to improve my language skills, and I hope this app will be good for me.

First of all, Hhi!

No capital letter for 'hi'.

I'm Yaren and I'm 20 years old.

I'm studying at prep school.

'study at somewhere'

I hope to improve my language skills and I hope this app will be good for me.

To use an infinitive after hope in this instance you need to use the to-form. I hope TO improve something.

First of all, Hhi!

Only use a capital letter when beginning a new sentence!

I'm Yaren and I'm 20 years old.

I'm studying in prep school.

I hope to improve my language skills and I hope this app will be good for me.