Bear in mind you need a fairly large space to lay even a moderately sized set out. I bought my kids a Thomas the Tank Engine Hornby set a couple of years ago and had to get an 6'x4' piece of chipboard to lay it on. Hornby do a few sets that come with a 6x4 paper mat with a printed layout on - you buy the first couple of sets of track, then add bits when you get them (full Thomas set was 5 track packs + 4 scenery packs, plus extra engines/rolling stock, etc).
Oh, and remember to go for double-O gauge - it's the most common, but there are several other model gauges available.
Something like
this is a pretty useful start. You start with a loop and a siding, but can expand it to 2 full loops, 3 or 4 sidings, and a couple of stations + scenery.