What do you want to write?. If is a text (for example "HELLO"), write the text > right click on the text layer > select "Create Mask from Text". This will give you a mask for each letter so you will need to duplicate the light emitter according to your text (for example in HELLO you will need 5 light emitters). Then do as Bill recommended, copy and paste the light path data into the light position property.
Note: If you are using Particle World you need to make an expression to link the X, Y and Z values with the lights position. Andrew Kramer discuss how to do this in the tutorial 3D Stroke Effect in Video Copilot.net.
The expression will look something like this