If you are neither US citizen nor US resident - you will be taxed only on income from US sources.There are no taxes on transfer funds - you might be only taxes on income earned. How the source of income is treated - see IRS publication - http://apps.irs.gov/pub/irs-pdf/p519.pdf
Basically, the earnings will come when selling the stocks. Will I then be taxed on that revenue?
yes - if these are stocks of US companies
Perfect. Last question. In that case, as a reference, do you know the approximate % to pay?
depending on the total income - you might be required or not to file your tax return based on your total income from US sources. Please also be aware of US-France tax treaty - income from the sale of stocks is a capital gain - if you are a resident of France - it will not be taxed - see IRS publication page 36 - http://www.irs.gov/pub/irs-pdf/p901.pdf - see the row for France. You will need to file a form with your broker to claim the tax treaty exemption.
This is a form to use - Form W-8BEN (PDF) Certificate of Foreign Status of Beneficial Owner for United States Tax Withholding - but consult your broker to be sure there will not be any withholding.