Am selling a business. How to determine capital gains? How to determine amount of tax owed?

We need way more information than has been provided:

• Entity type
• Type of sale; asset, stock
• Was there a purchase agreement
• What type of assets were in the business
• Any covenant not to compete involved
• Most likely additional questions will come up as information is provided
• Finally, it may be in your best interest to have a one on one with a tax professional at some point

iI would suggest you seek the advice of a tax pro who can go over both your personal and business situations to give the best advice for minimizing your tax burden. 

Agreed ... using a local professional would be wise ... you have to fill in and file  a 8594 form  that both parties agree on and shut down the business if needed ... these things can be compicated if you don't know  what you need to do.

https://www.irs.gov/newsroom/what-if-i-close-my-own-business

https://www.irs.gov/pub/irs-pdf/f8594.pdf