If you are a US citizen, permanent resident (green card holder) or resident alien (living in the US and pass the substantial presence test), you are required to report and pay income tax on your worldwide income following US tax laws. If you also pay foreign income tax on the same income, you may qualify for a deduction or credit on your US tax return for those taxes.
Under US tax rules, you are not required to disclose ownership of assets, except that you must disclose if you own or control foreign financial assets like bank accounts. (This is not a taxable situation, but you must disclose ownership of foreign bank accounts with assets more than US$10,000 or equivalent.)
Your capital gain is the difference between the selling price and your basis. Certain adjustments are allowed to both the selling price and the basis. For property you purchased, your basis is the original purchase price. Basis may be adjusted upwards by the costs of permanent improvements such as a new roof. Basis may also be adjusted downward if you use the property in business and claim or are entitled to claim depreciation. You can reduce the selling price by subtracting certain costs of selling, such as real estate commission and certain taxes and fees you might pay on the transaction. For property received as a gift, the basis is more complicated to figure out. Property that is inherited usually has a basis equal to the fair market value on the date the previous owner died. You can learn more about basis here.
https://www.irs.gov/taxtopics/tc703
Basis has nothing to do with the value when you immigrated. Basis is also not adjusted for inflation (even though some other countries allow for an inflation adjustment. If the property is in one of those countries, you would calculate your capital gains and pay tax in the US using US rules, and you would calculate your gain and pay any local tax using local rules.)
