I am a newbie myself and actually asked the same question as you on the 'scale' forum.

First and foremast you only glass over sheeted surfaces - not surfaces with large gaps. If your plane has open bays in it's constructions or a frame of some kind, it is not suitable for glassing. You will need to sheet the model with balsa before glassing. Glassing looks to be much more difficult and painstaking than covering. From what I have read it will give you a nicer and longer lasting finish and most importantly it gives you a finish that can be painted.

Here are some instructions on how to glass that I found:

[link=http://airfieldmodels.com/information_source/how_to_articles_for_model_builders/construction/apply_fiberglass_finish/index.htm]Fibreglassing tutorial[/link]

I would love to try it one day but I doubt it will be any time soon I don't have the space to do it in..

Photoniq