Here are the instructions for searching by citation count on Google Scholar:
1. Search for something you might be interested in (or a paper you’re familiar with)
2. The results will show up with a small link, on the lefthand side under the text which says “Cited by #”
3. Click on this link!
4. The results will re-sort so that the highest citation count papers will be at the beginning.
Let me know if you have any questions!