Q.putting weed killer on garden in the fall
I have been told that it is good to put weed killer on the garden in the fall and that it will help control the weeds next year. Is there any truth to this?

Yes, applying a pre-emergent in the fall is the best.
You can apply any time now through November.
Here are some links for you with more information. Always follow the application directions for the specific product that you use.
https://www.gardeningknowhow.com/lawn-care/lgen/lawn-care-tips-for-the-fall.htm
https://www.agry.purdue.edu/turfnew/pubs/ay-9.pdf
https://www.gardeningknowhow.com/plant-problems/weeds/pre-emergent-herbicides.htm