Inorganic chemistry

I have this exercise for my inorganic chemistry course; for NO3- show the creation of π molecular orbitals and how they are claimed by electrons and draw the π bonded molecular orbitals. The thing is we've only done a similar process for compounds like F-F, so I have no idea how to do this. Also, I have to do the drawing on computer and not by hand, anyone know any free websites or apps I could use for this?