What a huge subject to think about. I think one of the most memorable projects I was ever involved with was when my Dad decided to build us(him?) a railway layout when I was a young teenager.
The things we learned from that over the years was immeasurable and it played a big part of my modelling developement.
As for a starting point the biggest dictating factor is space. When you determine how much space you can allocate to giving away to your layout you then have to decide how much of an area you want your model to portray balanced against how much detail you want to display.
For the vast amount of indoor layouts there really is only HO/OO or N gauge to give you access to a wide range of roling stock and accessories.
Check out this link:
http://www.totalmodel.com/workshop/pages/workshop_219.shtml
However most people only go for N gauge if space is quite limited and HO/OO gauge has by far the widest range of accessories available to enhance the layout.
Once you have decided on the scale the next step is to decide on the type of layout and this is really dependant upon what you want to model. Some people go for a direct copy of a particular stretch of real line whereas some go for a large shunting yard only. Popular is a combination of a shunting yard with either one or two large circles around it to give you a bit of main line traffic as well as a bit of shunting.
Once you have a basic idea you then need to make some basic constructional decisions such as:
What type of track do you want to use, flexible (Peco) or a modular system (Hornby)?
Do you want to have remotely operated points?
Do you want to incorporate remotely operated accessories such as lights in buildings etc?
Do you want to incorporate buildings and if so what kits to go for i.e. card (quickbuild) or plastic (
Airfix)?
Lots and lots of things to think about in the purely planning stage but I can't emphasise enough the need to pre-plan. It is far more difficult to electrify points after the layout has been built than it is to lay the wires in as you go along.
I hope this gives one or two points to consider but the more you think about it the more you will come up with.
One of my biggest regrets is not having anyone around to build one for nowadays!!