As a nonresident alien in my first months, am I taxed only on my U.S. income?

Generally yes. While you're a nonresident alien, the U.S. taxes you only on U.S.-source income and income effectively connected with a U.S. trade or business, filing on Form 1040-NR. Wages for work performed in the U.S. are U.S.-source and taxable; your foreign bank interest or home-country salary earned before arrival usually is not. Certain U.S. investment income like dividends may face a flat 30% withholding unless a tax treaty lowers it. Once you pass the substantial presence test or get a green card, this narrow scope expands to worldwide income. Understanding which side of that line you're on drives everything. The IRS describes nonresident taxation in Publication 519 at irs.gov.