How can I make a tree on the ceiling using a garland and lights?

I'm not really sure what you have in mind, but if you want it to hang from the ceiling, start with a hook in the ceiling. From there you can drape the garlands and lights to the wall.

You could hang the garland and lights with clear fishing line and make it in the shape of a spiral.

If you are talking flat to the ceiling, you could use spray adhesive on the garland and stick it to the ceiling, and use some cable staples that have a padded area to not cut the lights accidently..... push or tap them in, but I don't know where you plan to plug it in?

You could use sticks to make a tree shaped frame similar to what we did here

https://www.hometalk.com/diy/decorate/rooms/rustic-twig-chandelier-23391555