Start by reading the notice slowly, carefully and thoroughly to see what exactly the IRS is saying. The notice won't give the exact details of every calculation, but it will say "changes to your income" and "as corrected by the IRS". This will show the income the IRS thinks you failed to report.
https://www.irs.gov/individuals/understanding-your-cp2000-notice
https://www.irs.gov/newsroom/irs-explains-cp-2000-letters-sent-to-taxpayers-when-tax-return-informat...
https://www.irs.gov/taxtopics/tc652
Then, go to your online account at the IRS and get your wage and income transcript for the year in question.
https://www.irs.gov/individuals/get-transcript
Review it to see what the IRS has file for you, and try to figure out why it is different than what you reported.
It might be that you had income you forgot to report, like sale of stocks or a retirement withdrawal; or it could be that you had income that you thought was not taxable that the IRS thinks is taxable, like the sale of your home. Or you could be the victim of identity theft and someone issued a 1099 in your name that was really for someone else. Or a business might have issued a 1099 with the wrong SSN due to a typo, or a data entry clerk at the IRS might have entered the wrong amounts when typing in some form.
If you can find the error in your income transcript, you would reply with proof of the error.
