The first layout should be a balance between cost and fun. It needs to be more than a simple circle of track which can quickly become boring. However, even if you have plenty of money, I don't recommend going all out immediately. Find out whether you and your family members like it.
I recommend something like the above Lionel suggestion. The cost of the track for the green and magenta parts is $115. I've added an industrial siding (orange) which might cost another $50 or so (the switch is the expensive item). You could cut the cost considerably by purchasing used track that many model train shops sell.
I would also start with a ready-to-run set made by Lionel and by MTH. They include transformer, track, locomotive(s) and cars. Make sure the set has an operating car or two or purchase one separately. Some of these sets are outstanding values as the manufacturers want to get more people into the hobby. I would also make sure the locomotive comes with sound (i.e., whistle, etc.) to add to the realism and fun.