Select File --> Export... from the menu to export your Audacity project.
Exporting compresses all your tracks into a single file for playback on a computer or mobile music player. The most common formats in use for exporting are MP3, Ogg, or Wav files. You cannot easily edit an exported Audacity file again in Audacity as it will be flattened into a single track. It is always a good idea to save your work as an Audacity file before exporting; see: Saving.
After selecting the Export option you will be asked for a filename and the filetype to export your project as; this is where you will select either MP3, Ogg, or Wav. The most commonly used file format for upload onto the internet is MP3.