What type of sharing do you have in mind? If you are talking about illegal sharing of copyrighted music, this isn't the site for such information.
If you want to share your iTunes music library with others on your local network, the feature is built right in. In iTunes->Preferences->Sharing, check "Share my music" and indicate whether you want to share all playlists or only specified ones. You can also control the name under which your music appears and require an optional password, which is a little odd since it's only for your local network, but up to you in any case.
iTunes is free software so anybody on your network can use it. They should check the "Look for shared music" checkbox in iTunes->Preferences->Sharing.
Assuming you have files you can legally share with friends or others over the Internet, if you want to share files over iChat, you can only copy them to/from other users manually. I couldn't tell from a quick look whether iChat's Applescript support includes file transfer. You could also look for other IM software or consider setting up an FTP server or web site with files you want to post for others.