ok so i've made many cakes before but would like to make a special decorated cake
where do i start?
can i but some supplies like fondaut at supermarkets?
can anyone please suggest a easy idea
muich appreciated! 
Don't know how much experience you have other than baking cakes, but if you want to try out new things, and are knowledgeable, there are so many books out there to look through. Yes, grocery stores carry 'limited' supplies. Cake stores near you, Michaels, JoAnns all carry supplies. On CC they carry everything as well as looking up recipes here for things like fondant, etc.
Get a theme and go from there. Good Luck! If you run into any questions this site is your answer!
The cake most people start with is either a sheet cake (9 x 13 pan) or 2 -9inch layers with buttercream in between to 'glue' them together.
If it's for a child's birthday you can use candy to make borders and designs (think gingerbread house).
Use an easy recipe for the frosting like the wilton class buttercream:
1 cup Crisco
1 teaspoon vanilla
2 - 3 Tablespoons water
4 cups (1 pound) powdered sugar
1 Tablespoon meringue powder (if you don't have this, leave it out)
Cream Crisco, vanilla and water. When mixed, add 1 cup at a time of the dry ingredients, mixing well between.
This will give you a medium consistancy frosting, enough for a 2 layer cake, or a 9 x 13.
To make it taste better you can use butter for part or all of the Crisco, but it will make it yellower.
Remember if you have questions, ( even ones you think are dumb!)ASK THEM!!! None of us knew this stuff at birth! 
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