First Year Abroad
You should know first and foremost that all US citizens are required to report their worldwide income to the IRS on their tax return. The two primary forms required for expat returns, in addition to the usual Form 1040, etc., are Form 2555 (PDF) – Foreign Earned Income Exclusion (instructions) and Form 1116 (PDF) – Foreign Tax Credit (instructions).
You are also required to pay into a social security system. The US has Totalization Agreements with countries to ensure that you are not having to pay into two systems, but you must pay into one.
US citizens who live abroad get an automatic 2 month extension to file until June 15th. However, if you want to extend this until October, you will need to file for an extension by April 15th. Make sure to check with your state to see if they accept the federal extension or whether you need to file an extension with the state as well. Filing an extension is never a bad idea for expats— it gives you additional time to decide what you are going to do.