Well you really need a midi keyboard to input your music to play along with your fiddle. Otherwise, you will have to enter in each note manually in the key editor with the draw tool. The manual under Help-> Documentation is a good place to start learning the basics.

You will need a VSTi that has a piano sound in order to do this (or a hardware keyboard with a piano sound). LE1 does no come with such a plugin. In this case, you would have to purchase a piano plugin. If you search the internet for free VST plugins you might find a free piano VSTi as well. Also know that your Garage Band piano is strictly for use in Garage Band.