Implementing a "share" action is easiest if you are targeting API level 14+ (Android 4.0), as documented here: Adding an Easy Share Action.
If you need to target an earlier API level, the process is a bit more involved, as documented here: Receiving Content from Other Apps.
(Note that while I normally consider link-only answers on Stack Overflow to be a poor choice, the process of implementing "share" actions is somewhat involved and fully documented on the Android developer site. This is a case where simply pointing you at the documentation seems like the most appropriate thing to do.)