how do you throw snowballs...im super lost

Go outside and roll some snow into a ball with your hands. Make sure you're wearing mittens. Then throw it at your friend and yell "GIFTED MOTHAFUGGA!"

There will probably be a big button in the shop or on the client's main page after the snowdown events begins.

I believe you can't until snowdown actually starts (the 10th), and it should be available through the normal gifting route.