Well, from somebody who just finished his first layout design in O gauge I can assure you that you don't need a computer.
Go the center of the floor and put painter's tape down, in the outline of your chosen layout board dimensions.
Buy a bunch of really cheap, old, rusted track and switches in the gauge and circumferances you want, get down on the floor on your hands and knees in the middle of the tape area , with your tools and track saw, and just start cutting and fitting the stuff together in a creative fashion that you like. Design as you go. Start with the inner sections first, as they must be tight enough to fit inside your outer curves and track. What you are doing here is building a cheap template, just to get the layout planned and properly fitting together. The tape outline shows you how to keep it inside the dimensions of the layout board.
Yes, you will do alot of head scratching, and shuffling, and moving the small curves and loops around to get everything to fit, as well as cutting some straight and curves to custom lengths, but when you are done you will have a unique layout of your own design. (Unless you are really up to speed on the electrical aspects, make sure you don't accidentally build any reverse loops! ) Hook up a transformer and use a test engine and cars to make sure they stay on the tracks and switches.
Then, go piece by piece through your track, throw out the really bad pieces, clean up thoroughly the so-so pieces, and buy brand new pieces as needed to complete it.
When done, then build your layout table, move your track onto the table, and screw it down. If you are going to put down cork bedding, then put the layout down on the table, trace the track, then remove it to put the bedding down. Then put the track back on, and screw it down
I don't know how old you are, but if you played with Legos as a child, did you need a computer or book to show you how to build a neat custom building on the building platform? Nope.
For me, copying a layout from a book or computer printout, and then just buying the list of track pieces they tell you to buy, and then just snapping them together, isn't building a railroad. It just snapping a plastic model together.
To each his own.
Have fun.
Mannyrock
