Visit your local hobby shop to become acquainted with the different scale sizes.
Some hobby shops have small working layouts.
Available space for a layout will also influence your final decision. There are also some good instructional videos. Do some planning, but not to the point that you endlessly postpone building anything. Check out the wealth of information that is on the Internet for the scale and railroad that you plan to model. Learn from what other people have done, but do not be intimidated by it.