Only photos taken after turning on photo stream will be uploaded. If you want your existing photos uploaded you'll have to import them to your computer, enable iCloud syncing on your computer, and put the photos in the upload folder on your computer. To enable iCloud syncing on your computer see either PC or Mac here: http://www.apple.com/icloud/setup/.
You don't see your photos in iTunes. ITunes only syncs photos to your phone and backs up your camera roll photos in the iPhone backup. If you aren't syncing your computer iCloud with photo stream syncing enabled, you have to import your camera roll photos (and videos) to your computer in order to see them there, as described in this article: http://support.apple.com/kb/HT4083. If you do enable iCloud syncing with photo stream the photos will be located in the download folder, which by default on a PC is C:\Users\Apple\Pictures\Photo Stream\My Photo Stream. On a Mac they are in iPhoto or Aperture.
You may want to look at the photo stream faq: http://support.apple.com/kb/HT4486.