I forgot how to set an object to a path to get it to move. I selected Curve modifier and set it, but it doesn’t move, how do I move it again?

The easiest way is to Parent the Object to the Path. Just select the Object first and then the path with shift held down. Then press strg+P and a small menu pops up. Select follow path.
If you want to follow the path precisely position the Object at the beginning of the Path (before parenting!) and Apply Location.
Have Fun!

It doesn’t simply move along the curve when you start the animation. You have to move the object along your “Deformation Axis”, which you assign in the settings of your curve modifier.

Another possiblity (without a curve modifier) would be: click your object, shift-click the curve and press ctrl-P —> Follow Path. However, you can’t animate the movement of the object then. It will move with constant speed from the start of the curve to its end.

It’s suppose to be a roller coaster type thing, so what would be recommended for that?

As you surely want the speed of your roller coaster to change depending on whether it’s going up or down, the Curve modifer would be the right option.

I forgot, I just move the timeline, then move the object, and change position keyframe.