Doing an exchange program in high school can be a lovely way to experience day-to-day life in another country, within the safety of your "host family." But not every teenager is ready for that kind of adventure. If you're currently attending a university, call their admissions office and ask if they have an exchange program.
Because of increased security measures, the US government is very behind with issuing passports. Get yours now. Even if you don't plan to go for a while yet, it takes time and you'll be glad you have it. Go to any post office for an application. You'll need your birth certificate, driver's license (if you have one) two pictures and about $60, as well as your parents signatures.
Find out when your school's application time period is. For most programs, you'll need to apply almost a year ahead of time. For example, while in college, I applied to my Education Abroad Program as a sophomore. I actually traveled to the UK for my entire Junior year. Planning well in advance is necessary.
Do your best to put together a compelling application: get as many great references as you can, write a stellar essay, learn about the country you'd like to visit. You may not necessarily need to speak a foreign language to qualify. There are many English speaking countries. But if you do have a foreign language, mention that in your application. Also be willing to change your mind about where you'd travel. If you list several countries, you're more likely to be selected.
Once you get chosen you can put some serious plans into place. You will still be paying tuition, so however you cover that at home, you'll need to continue. Most countries will not allow you to work while in their country. If you normally have a job to make your ends meet, you'll need to take a student loan for the year. Trust me, it's well worth it.
Prepare as much ahead of time as possible. Really think through what you're going to take with you. Remember that electricity will be different, so anything that takes a plug will need an adapter. It's usually easier to buy a new one in your host country. Give some thought to where you'd like to travel over Christmas and Spring break. Buy tour books, language books, guide books....whatever you can get your hands on. Get ready, because you're about to have the most amazing adventure of your life!