The previous owner of the house neglected the garden for quite a few years because of health, so the garden was a disaster when I bought the house.
I kiilled all the weeds and bought some plants. The problem is the soil
has not nutrition. I don't want to use chemical fertilizer. Any suggestion
on how I can improve the quality of the soil?

Ok. I tilled and added some mulch, but forgot the top soil. The problem is not top soil because the soil on the surface is quite good from the mulchs of the past. When I digged a little deeper, I can see sand! Do you guys make your own compost?