今日は

↓

こんにちは

It's not that the kanji is wrong, but it's better you write 「こんにちは」in all hiragana because sometimes we confuse it with the phrase

「きょうは」,

which uses the exact same kanji, but means "today is..." instead of "hello"

So just be careful :)

--------------------------------------------------------------------------------------

はじめまして。
私の名前はめいです。
十四歳です。

No problems here, it's very coherent!

----------------------------------------------------------------------------------------------------

高校生の高校生です。
三年生です。

↓

高校三年生です。

You're being redundant by saying 「高校生」 twice...

If you want to say that you're 3rd year senior high school,

say

「私は高校三年生です。」

 It's best to keep your sentences simple. :)

-------------------------------------------------------------------------------------------------------
よろしくお願いします/ どうぞよろしく。

There's nothing wrong here, but I'll give you tip so that you can sound as formal as you can :)

I'll take a few parts from both of the phrases you used...

よろしくお願いします

どうぞよろしく。

And add them together.

どうぞ + よろしくお願いします。

どうぞよろしくお願いします。

This is the most formal way to say 「よろしく」 which is the Japanese greeting for "nice to meet you, and I would like to build a good relationship with you".

Being formal is a good way to make a good first impression when talking to Japanese people, so I recommend that you use this over the others.

Overall, I like that you used the three basic phrases for self-introduction, which are...

★はじめまして。

★私の名前は…です。

★どうぞよろしくお願いします。

These are very important greeting used in Japanese, so always keep these in mind.

If you want to know more about these phrases and their meanings, you can refer to this YouTube video for reference. :)

https://www.youtube.com/watch?v=eSrpkd8X4MQ 

Good Job! And good luck learning more Japanese phrases!

頑張ってください！

今日は

はじめまして
私の名前はめいです。
十四歳です

高校生の高校生です。
三年生です。

➡「高校生です。三年生です。」 or　「高校三年生です。」

 
よろしくお願いします/ どうぞよろしく。

➡「どうぞよろしくお願いします。」

Hii everyone. I hope someone can correct my introduction on grammar spelling and kanji. 有難うございます。

大変よくできました。：）