The design of your layout will be governed by the space you have & naturally your budget. Some like to base the design on a real life exciting or old track that used to be. Some will base it around a goods yard or large station layout. I, like many others have made my design so suit my garden allowing for some expansion at a later date if wanted.
The base & Type of layout:
There are a few ideas on what to build the track on & at what level suits you. Some will remove the turf/grass to have a channel that can be lined with a garden weed barrier cloth that can be backfilled with a gavel to lay their track on. With this you can lay flexible conduit under the gavel for electrics if needed. Others will put in a foundation & build a block/brick wall to run the track on. You can also build some flower beds & run your track through a scene like a jungle or desert. Some of these might depend on the climate of where you live. For myself & my age i am going down the route of building a raised track to save being on my hands & knees & getting backache once built. So i have aimed at 24"/600mm high & because i have the room, around 3'/1met + in from any boarder to ease cutting of hedges.
Control of everything
I decided to go down the route of RC "radio control" trains that have onboard battery to power them & 12 volt to power for signals, lighting & points. The points will have a 12 to 5 voltage reduction as the servos i am using run on 5 volt. This will all be controlled via ewelink home automation system that is simple to use & very cost effective. "£8.00 a year" & a 4 switch relay at £6-£10. This will all run off of wifi via my laptop, phone or even Alexa. I have extended the wi-fi range with some TP Link Deco M4 at around £100 for three & now have great wi-fi in every corner of the garden & also garage.