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If it is just for your page:
FBML is a plugin that lets you setup HTML and FBML code on a tab on your FB page: https://www.facebook.com/apps/application.php?id=4949752878
A step by step: http://blog.hubspot.com/blog/tabid/6307/bid/6168/How-to-Create-a-New-Facebook-Tab-in-3-Easy-Steps.aspx
You can then customize that html with javascript calls to the FB api: https://developers.facebook.com/docs/reference/javascript/
You can use the Feed dialog for posting on the wall: https://developers.facebook.com/docs/reference/dialogs/feed/
If it is for an application users would install on their page:
FB has a step by step guides for creating an application: https://developers.facebook.com/docs/guides/canvas/
And just as before you can use the Feed dialog for posting on the wall: https://developers.facebook.com/docs/reference/dialogs/feed/