i made buttercream and its too thin i used all of the ingredinents and i dnt kno. is it cause i used 45% reduced fat butter nd lil shortening. i put marshmallow in it to thicken it worked for a lil but got bck soupy. how can fix it and also give me tips for next time.

What was the recipe you were using, and the method? Can you post it here?

Also, low fat butter has lots of liquid in it and it doesn't work for baking or for making buttercreams...unless the original recipe says to use it, you shouldn't have.

AAmerican buttercream nd I think I added too much shortening nd butter

Where did you get the idea to add marshmallows? If it was too thin I would have added more powdered sugar...and definitely never use low fat butter or margerine. Can you tell us your exact recipe you followed?

AYou mean like marshmallow cream?

Well, what you made was "your" recipe, not any known recipe for American Butter Cream (ABC). 

Generally, if you want a recipe to work then it's important to follow the recipe as written.  There are plenty of those here on CC in the recipe section and online.  This is a good source:

http://www.wilton.com/recipes/browse.cfm?cat=Icing