The first visit in the US refers to the very first time you entered the US. That could be an entry as a tourist or non-tourist (student, intern etc.)
If you cannot remember the exact date, you have a few options for how to proceed:
1. You can check your passport and see all the visits to the US marked there.
2. If you have changed your passport recently and do not have access to your old one, you can check your visits on your i-94 (if you have it on paper) or through their official site (here you can see your visits for the past 10 years only) - https://i94.cbp.dhs.gov/I94/#/home
3. If non of the above options work, you can put an approximate date here.
Please note that it is very important to list your very first visit to the US on Sprintax, in order to have your Residency status determined correctly.
N.B Step 1 Residency - Visa Details - In that section the "Date you first entered the US" is taken from the previous sub-step - Residency status where you have already listed your very first visit in the US. Do NOT change the populated date on the next sub-step.