I would like to know how to use buttercream in icing a cake.
i.e borders, shells, ruffles,zigzags,basket weaves, rosettes ,roses, leaves etc...
Are there any posts that have a tutorial on how to pipe using buttercream.
I would also like to know what tips to use for each decoration??

Go to Wilton.com and search techniques... a lot of the descriptions will also have a video available to watch.

Ditto: Wilton is Great, also surf Youtube for video tutorials. HTH.

If you're interested in all aspects of cake decorating, you might consider taking some classes or applying for a job in a bakery.

The Wilton site also has a chart that shows what each tip can be used for.