<<sorry I am very new here, and my question is very basic,>>

Howdy Ric...welcome to the folder. And since you are new to taxes and investing, you should really drift on over to the Taxes FAQ area for some breathtaking discussions on taxes, investments, and other issues.

<<the question is now that I have openned a roth IRA, I understand the tax on that,>>

Do you really? If you're not quite sure, you can check out my post regarding taxes and the Roth IRA in the Taxes FAQ area. It'll tell you all about how your Roth IRA distributions may (or may not) be taxed.

<<but I also open a mutual fund with $20,000 it is showing a 15% gain in two months..... I plan to let it ride for at least 15 - 20 years.... do I pay taxes on the gain, each year or it it only when I sell or cash in??>>

Mutual funds are strange animals. They are required to give you distributions on at least an annual basis. And those distributions will be taxable. And if you elect to "reinvest" those distributions, you will really be increasing your cost basis in those mutual fund shares. So you are going to have a BUNCH or recordkeeping to do over the next 15-20 years.

In addition, when you finally sell those shares, you'll pay tax on the capital gain on the appreciation that you received on the sale. So with mutual funds, it's a "pay me now AND pay me later" situation. That why we here are the Fool are so fond of individual stocks. You can escape the "pay me now" part of the situation.

But you can read more about how mutual funds are taxed in my various posts in the Taxes FAQ area. Additional, my new book also deals very specifically with mutual fund tax issues. So you might want to check it out.

TMF Taxes
Roy