Hiya Smith 368, and welcome. Here's a brief synopsis of WHAT there is to 'spot', and what each entails.
Locomotive numbers: dead simple, but locos are in short supply nowadays, and mostly used on freight workings, hence the surge in interest of freights over the past few years;
Electric & Diesel Multiple Units: a must, really, since most passenger services are operated by EMU and DMU nowadays; the Unit numbers will be found at each end. Here's where you hit your first decision obstacle: Most Multiple Units consist of a number of individually numbered coaches; I first started noting each coach number when I realised that occasionally (often on the SR!) individual coaches could be shifted from one unit to another, and it was the only way to ensure I was underlining what I'd seen, as opposed to what might be listed as 'the proper formation'. The problem with doing this is it's very labour-intensive, especially at busy stations, such as Clapham JUnction, London Bridge, Stratford, etc; AND - if you're intending to take photos of trains, it can all get a bit too frantic. Same thing with Loco-Hauled Coaches.
Departmental Vehicles/On-Track Plant: a wide variety of odd looking things which operate everywhere in order to do track repairs and the like.
Wagon Numbers: VERY labour intensive, especially if collecting numbers of RIV wagons (which run to/from Europe) - lots of digits to write down for each wagon, often being renumbered, and can be very confusing. Lots of people DO collect wagon numbers though.
Haulage Bashing: basically, making a mammoth effort to be hauled by/travel in everything possible; some 'bashers' include all units in a formation, some just the unit they've travelled IN, some even include the actual coach of the unit they're in.
Underground Trains: numbers on each coach in fixed formation (unnumbered) units, so very busy, as are most of the platforms, and the trains themselves.
That's the basics - I'd suggest you start with Loco and Unit numbers, plus maybe Loco-Hauled coaches (not too many in everyday use, mostly charter stock on 'Specials' etc), and see how you get on with that.
I see you're in Sheffield; start off there, it's quite busy, as you probably know, and Doncaster's good for most of what I listed above. Good luck, and keep us informed of your exploits and experiences!
