Is there any particular sort of robe you had in mind? Do you want your robe to have a hood? Do you want it to wrap around, to button, to be all one piece? Do you want your robe to be of a particular material?
Fabric stores have tons of patterns, and they usually have books that you look through to find a picture of the one that you like best. That's a good way to start if you aren't sure what sort of style you want.