You can be thankful for stuff without any deity or higher being in your life. If you must have someone to articulate it you can thank to fate, chance, blind luck, other people or your past self to brought you to the things, places, events, persons you are thankful for.
In my experience it's a good exercise to introduce positive thoughts in life. I also like to think about the good things that happened to me on the end of the day - sometimes I need several minutes to bring up a single thing but practice makes wonders.
I used to be like you, and I respect your opinion, and while I still do not believe in any higher power, I have come to realise that gratitude — for being born on planet Earth, and in a developed country, and not on Mars, if for no other reason — is powerful and helps me whine less, work more, have a more positive attitude and think about what I can do for others.
