Thank you notes can some times feel daunting. When I was younger I always hated having to write them because I never knew what to say except "Thanks!". With practice I have gotten a little better and I have come up with a crutch for those times I'm not sure what to write. I just remember my G.I.F.T.S.
Dear so and so, or just their name is fine.
I- Include examples
If you received a gift you could say "I love my new scarf! I am always looking to add to my collection." If you are thanking someone for a favor, "I would have never been able to pack my apartment up with out the boxes you brought by."
Mention the future in some way you could mention how you are going to use the gift you received or how favor etc will influence you. . Ex: "I can't wait until this winter to wear it with my new coat."
End your note expressing thanks.
Clearly, it doesn't need to fit exactly but I always find it helpful to have a few points to make if I am having a hard time. It's the thought that counts and your thank you note recipient will be delighted whatever it says.