All you need is a passport from the U.S. state department, and most likely a visa from the country(s) you will be staying in. there is no U.S. law saying how long an american can reside outside of the country. but most foreign countries will require you to have a visa. most countries in the european union (UK, Ireland, Sweden, Norway, France, Spain, Portugal, Italy, Germany, Austria, Netherlands, Denmark, Belgium, Luxemburg, and the more recently admitted estern bloc countries poland czech republic, slovakia, hungary,slovenia, etc etc) you will need a visa if you plan on staying in one of those countries for longer than 2 months. other countries such as russia, you will be required to have a visa along with your passport before you enter the country. you obtain a visa from the embassy (of the countries you are visiting) or nearest consulate in the united states. every country has different rules and regulations in determining who they grant visas to and it is not considered a right. typical kinds of visas are: work visas, tourism visas, and student visas.
generally to apply for a work visa you will have to show that for some reason an employer in the country you are wishing to work can not find a suitable national/citizen to fill the position for what you are applying for. in europe, you generally do not need to apply for a visa prior to entering the country, you can do so afterwards at a city hall (in germany, anyway). in all actualaty you do not really need a visa, and you could theoretically stay in germany or england for that matter indefinately. the problem is how would you support yourself? most employers will not hire you unless you can show you have a right to work in the country (thats what a visa is for). but it does happen. one thing to consider however, is they can and most probably will deport you if the authorities ever find out. Americans living abroad in europe for example though, will have a much easier time keeping a low key than citizens of third world countries (and avoiding scrutiny of the authorities). and although i don't think it is a violation of any american laws to violate immigration laws of another country, it would be extremely embarrassing for you to be deported back to the united states, so I strongly suggest getting a visa. another thing to consider: by living abroad (even without renouncing your citizenship or allegiance to your country), will most likely brand you as an "expatriat" (expat for short). The longer you live and reside in a foreign country, the stronger this association will be. This may or may not be a factor in your life, so long as you don't have ambitions to work for the U.S. defense industry or various intelligence agencies, as part of the background check that they will do is to take into account your foreign ties.
some additional things to consider. to obtain a student visa in Germany (probably similar to other European countries as well) you need to show that you have been accepted at a german university AND you need to show that you are financially stable for the duration of your stay. this can be done with a bank statement showing that you have sufficient funds, or I believe you can get a sponsor (a german citizen) to sign for you claiming they are supporting you. a great thing about studying in germany (i think its the same in france and in england) is higher education there is free to the student. there is no tuition charge for germans, so there is no charge for americans either.
this is important: i've seen a lot of disinformation on the net concerning travel requirement to and from Canada/Mexico. it is the official policy of the U.S. State department and border enforcement agents to require all US citezens (and anyone for that matter) entering this country by land or sea to have a passport. (this did not use to be the case, prior to about 5 years ago you could travel by car to and from canada for instance with only a drivers license). By january 2008 everyone (regardless of citizenship or nationality) will be required to at least have a passport (visa too depending on your nationality) for entering the country by air, even from canada and mexico. due to heavier than predicted applications, the state department waived this new requirement, but the waiver is only valid to passangers returning by air. (air passengers still need to show valid photo i.d., and proof they have applied for a passport). any information you hear saying you do not need a passport for returning to the united states by car from mexico or canada until 2008 is to be considered false and misleading. I can not vouch for individual border patrol guards at various crossings, but if it is the case that they are not controlling 100% of travelers, you need to be aware that this is not national policy.
because your friend may have vacationed in mexico without a passport doesn't mean you will be denied entry to mexico when traveling by car. and whats worse, if you are not controlled heading down to mexico, but you are when coming home, and you don't have a passport? can you think of anything worse than being stranded in mexico without a passport?
Lived in Germany for 6th months