I'm sure the pros on this site will know of several places to download software but I'm a beginner and I'm using Anyrail. www.anyrail.com
It's free (I love that; free is good!) and intuitive. Just enter the size of your layout, the brand of track and type, and you can drag 50 pieces of track onto the work area to form any plan that you like. You can experiment with different radius curves, etc.