Choose your era, what "time" and "type" of trains do you want to model?

There are a couple of schools of thought on this. One is to just buy whatever looks good to you, and the other is to make things "fit" together in time and application, more like a real model of a railroad. Either choice is fine.

Why is this important? Because the decisions here affect the track, power, size of the layout, basically everything.

This should be one of your first decisions.

What era and style determines the size of the locos and also the length of the trains.

Do you want to model a narrow gauge logging or mining operation? Sharp curves, 1:20 locos, short trains, etc will drive your decisions on track power, type of track, minimum radius, grades.

Similarly, do you want to model the last great days of steam? Huge articulated locos, long passenger trains, multiple diesels pulling a long passenger trains, etc. will call for broader curves, probably DCC power, grades 2% and under, and just more space.

Try to determine what you want, it will drive a lot of decisions that are fundamental to your enjoyment of the hobby.

Many people like the "transition" era, where steam and diesel were both in use.

Don't:

A common mistake is to buy a starter set with sharp curves and a small loco, then extend the layout with the same sharp curves, and then become frustrated when your "next step" fails because you cannot get those nice passenger cars, or that big steam loco around the sharp curves. 

Starter sets are just that, and normally the track is worthless in a mature layout, keep it for under the tree. In 1:29 scale, the Aristo and USA Trains starter sets have good quality locomotives that will last you a lifetime. The $50-$100 starter set from Wallyworld won't last and will be the poor stepchild. Don't go cheap, you will be throwing your money away.