Happy birthday to my friend.

Happy Birthday to you!!
I'm sorry I forgot to heard your birthday. (<-これはちょっとモヤモヤ感じです！)
You became 51 years old?
I think 51 years is still young.
You looks always to happy. =>You look happy all the time OR You always look very/so happy.
It is fun and I hope a year of full for you. (<-これもちょっとモヤモヤ感じです！)

Have a fun time with your friends today!!
○×○×

こんにちはヒロコさん！あなたはとても上手ですね！がんばってくださいね　（＾＿＾）

Happy birthday to my friend.

Happy Birthday to you!!
I'm sorry I forgot to heard your birthday. 

Tabun you meant "remember" oboeru => hearing someone's birthday ha henna eigo desu :) 

You became 51 years old?
I think 51 years is still young.
You looks always to happy. 

You always look (very) happy.

When you say someone looks too happy, you mean it's no healthy to be so happy or you don't want them to be so happy... ja sore mo chotto henna eigo.

It is fun and I hope a year of full for you. 

I think you meant to say: "I hope it will be a year filled with fun for you." or  "I wish you a year filled with joy."

Have a fun time with your friends today!!
○×○×

(p.s.: Since recently I'm a certified English teacher, so I think I can probably help you improve your English. Nihongo oshiete kudasai. LINE to Skype ga arimasu :) yorosikuonegaisimasu )