Letter In Japanese

*Note: I am trying to write a letter in Japanese for my friend. She is also studying Japanese, and we are very close. The reason it is all in hiragana is because she does not know any kanji yet, and know very little. I also had a bit of a problem on how to start of the letter, I felt like it would be too formal to write something else than "To Eli,".*

エリへ、
はじめておてがみをかけて、ほんとうにしあわせです。おげんきですか。あたらしいいえおめでとう。にほんごのべんきょうはどうですか。がんばって！またおへんじをたのしみしています。

Did she move to a new house, or did she build her own house? :)

When I write letters to my close friends, I always use "◯◯ へ"

Hope it helps!

Letter In Japanese

*Note: I am trying to write a letter in Japanese for my friend. She is also studying Japanese, and we are very close. The reason it is all in hiragana is because she does not know any kanji yet, and know very little. I also had a bit of a problem on how to start of the letter, I felt like it would be too formal to write something else than "To Eli,".*

エリへ、
はじめておてがみをかけて、ほんとうにしあわせです。おげんきですか。あたらしいいえおめでとう。にほんごのべんきょうはどうですか。がんばって！またおへんじをたのしみしています。

Besides the header other than エリへ as you said, it seems a little bit formal for a letter to close friend but I'd recommend you as a foreign learner such a formal style to acquire decent diction than too ripe anime-tik one.  A suggestion I'd like to offer: separating words in the text with spaces like books for Japanese children as following,

はじめて　おてがみを　かけて、ほんとうに　しあわせです。　おげんき　ですか。　あたらしい　いえ　おめでとう。　にほんごの　べんきょうは  どう ですか。　がんばって！ また　おへんじを　たのしみに　しています。

It's easy to parse the text even for native kids.

As for the header, 「エリさん、」 is better than 「エリへ」 and veratile expression.

ex:

鈴木さん、

blah blah....

ご担当者さま、

blah blah ...

お母さん、

blah blah ...

The header を　エリへ　で始めると：

エリへ、

元気してるー？　私も元気だよ．．．

のような文が続く語感となります。　close friends だけに使える砕けた表現です。