First of all pick out a spot or two where you think the layout might look good in the yard. Try to plan it so it won't be in the way of any major activities that you and your family might have outside during the warmer months of the year.
Try to determine what type of engines and cars you might want to run.
This will help you decide how much room you should allow for your turns.
The minimum size track you can use is a 5' radius for your turns. So that means if you were to install just a circle without any straight track, the circle at any given point would measure 5' across from the center of the track, to the center of the track on the other side of the circle.
The larger the turn the better off you will be in the long run. For this will reduce a great deal of headaches in keeping the cars from derailing so much. The track should be very close to level from side to side, especially in your turns. But if you are going to bank your turns a bit, always keep the outer part of the track higher than the inside portion, as gravity will take over and keep the train on the track.
It is always best to start out slow. You can always add to your layout from time to time. Also before you start any yard construction, you will be much better off buying your track, and perhaps a starter set, meaning a relatively inexpensive engine and cars. Don't make your project too overwhelming at first, for it can turn into that quickly if you are not care full.
If I can be of any other help, please just drop me a line via e-mail.