How do I add effects to happy new year message

First, you need to know that in order for the recipient to receive effects in messages, the person must be using iMessages.

You can find the Happy New Year effect if you press and hold the "Send" button inside the message to the recipient. At the top you can choose Bubble or Screen. Select Screen and swipe until you find the party fireworks and press send.

You can get more tips for effects here:

Use message effects with iMessage on your iPhone, iPad, and iPod touch - Apple Support