If you have already digital real estate.
Check Google Analytics, Webmaster tools, … this will leave you some keys.
Also if you did run some Google Ads, in your “search term” report, you’ll find interesting data.

If you have none of the above, we should do some guess work. Luckily there are some tools (free and paid to help us)

The Keyword Shitter tool.

Google Keyword Planner (you might need an Google Ad account where you did spend some money recently).

Spy on Ads:
Google Ads
Facebook Ads