Write your address on the top left corner of the envelope. This is important because it tells the post office where to send the package in the event it cannot be delivered to the intended recipient. This should include your name, then the street address just below that, with the city, state and zip code written underneath the street address.
Affix proper postage to the top right corner of the package. For an envelope, one of the most recently issued postage stamps or a "Forever" stamp will suffice. For a package, you can use a postage meter or take it to your local post office to determine correct postage.
Write the address you wish to mail the package to in the center. This should be written with the name of the recipient first, then the street address just below it. Under that, write the city, state and entire zip code.