First choose your scale: realistically, you only have N and 00 to choose from. N-gauge gives you more in a given space, but is a bit more fiddly and less suitable for a beginner, although if you have some modelling experience, even if only Airfix kits, you should get by. 00-gauge is more robust and tolerant, but takes up more space. There are more models available in 00, although N has caught up in recent years; in particular, there is an enormous variety of kits in 00, but not so many in N.
I would suggest that you get a copy of a magazine so that you can browse the adverts to see what is available: personally, I feel that Railway Modeller (*) would be the best choice. Once you have chosen your scale, you could get a few catalogues or haunt a model shop. One catalogue I would suggest is the Peco Model Railways one: Peco is primarily a track and electrics manufacturer, although they do other equipment, and the catalogue contains a lot of useful information for planning the layout. You could also get one or two of their track planning handbooks, particularly those by the late C.J.Freezer: you may not build any of the layouts in them, but they will give you a good idea of what you can and can’t do in a given space.
* - Available from all good newsagents, some pretty average ones and even a few utterly appalling ones.
