Q.tilling heavy clay soil
I have heavy clay soil. Can I till it over? There is grass and some weeds. I would like to just till it over add mulch and sand. Will that help the sitting water issue we have in our yard?

Before doing too much tilling I would work on correcting your soil first. The information in the article link below will help improve your heavy clay soil and improve the drainage of your soil.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/how-to-improve-clay-soil.htm