I always have to write thank-you notes to my relatives for birthday and Christmas presents. Even if you don't have to, people love thank-you notes, and they'll put you on their good side. Here's some tips from around the internet (and from people) on how to write the best possible thank-you note. Your grandparents or aunts and uncles will absolutely love getting thank-you notes from you in the mail!
1. Be specific: Know who gave you what, and thank them for that. Don't just say "Thanks for the present you gave me".
2. Give an example: Tell your grandmother that you're wearing the sweater she knitted you as you're writing this letter. Or that you and your brother spent 45 minutes playing with the new soccer ball that your aunt gave you.
3. "Great to see you!": If that relative/friend/person was at the party/event/etc, say that you loved seeing them. Talk about something that happened at the party. Compliment them.
4. Look to the future: Write about how you can't wait to see your cousins at the family reunion/birthday party/ holiday.
5. End with regards: "Yours truly", "Sincerely", "Love", or just a dash (-) to sign your name at the end.