1. You do! The postage depends upon both where the letter is going and upon the weight of the letter. For a letter to France, the postage for a one ounce letter is $0.98 (three regular stamps, or 2 regular stamps plus 5 two cent stamps). You cannot use "first class forever" stamps for international mail. A one ounce letter is about two normal pages; if your letter is longer than that, you might want to use the 2 ounce rate of $1.82 (5 regular stamps, or 4 regular stamps and 3 two cent stamps). If you have a kitchen scale, weigh the letter before you apply the stamps. Don't use a grocery store scale; they're horribly inaccurate.
2. The country should be written by itself as the very last line of the address. Also, the post office prefers that you write the entire address in CAPITAL letters. Handwriting is taught differently in different countries, and capital letters are far more similar across borders than lowercase.
3. As long as you're sure that the letter has enough stamps on it, there's no need to go to the post office, unless you want any special services like delivery confirmation. These generally cost a few extra dollars, though.
4. Make sure you put your own address in the top left-hand corner of the envelope, just like you would for domestic mail. You don't need to write your address in capitals, since the only postal service that would use it is ours (if there's not enough postage or if the letter is returned), but you can if you want to. Also, there are a few guidelines for what kind of envelope to use: it can't be less than 5 1/2" by 3 1/2", or more than 11 1/2" by 6 1/8", and the sealed letter can't be more than 1/4" thick. Most regular envelopes fall within these guidelines. Try to use a white or lightly colored envelope; if the envelope is too bright, it will confuse the mail sorting machine, and your letter may be returned.