Hi Ash,
David and Jim have made very valid points on where to start. Quite rightly neither mentioned a choice of layout, that's quite a bit further on down the road. There is a good choice in N Gauge and there are several makers of the stock. You might like to look at the online catalogue of Graham Farish and Dapol, just to see what you like the look of, just remember, you don't have to buy them all.
I simply bought 2 sets which had trains, similar to ones, I had been on, one set also had a simple DCC {Digital Command Control} Controller and track. When you're looking at the catalogues you may find something which catches your interest. Another place to see what you might like the look of, are the very pages you are reading on this Forum. There are loads of different locos and rolling stock to look at so something there could well catch your interest. If you are doing that, don't stick to N Gauge, as it's the rolling stock you are looking for.
This article might give you some help about how to go about it...
How to choose which model train set to build
Magazines are a great source of information, with pictures and lots of adverts, showing what's available.
As the others have said, it is a really good idea to look at what interests you, before deciding to part with those precious pennies. Don't forget that people on here are only too willing to try to answer questions about whatever catches your attention.
Julian