It's probably worth pointing out that many sites, especially the non- "First Party" (TOC to us) ones, are quite unreliable with their prices because they include booking fees, which in theory amounts to their commission. So the price you would find on the South West Trains site should be the most accurate, and fairest, and failing that National Rail Enquiries. In terms of the route, you would need to change at Weybridge, as advised above, but the train will probably be advertised as not continuing to Waterloo when in reality it does. So if you hear one announcement on the platform at Weybridge, and another on board the train at (or after) Weybridge, don't worry about it; you will still end up at Chertsey. They never seem to update on board in the same way that they do at stations.
You'd have to be blind to get lost at Weybridge. When I was last there, the Chertsey train was advertised as a through service to Waterloo via Hounslow.
There are only three platforms, and it is an exceptionally straightforward change: literally all the OP will need to do is alight from the Woking stopping service (usually a red suburban train), hop over the footbridge and join the train standing at the bay platform (usually an eight- or twelve-car blue "Desiro" train.) A bay platform is one with buffers.
(As the OP is going to Chertsey, though, it would be wise to join the front 4 coaches, as the station has short platforms. Remember, the front four will be the four furthest away from the buffers - it sounds silly, but it's astonishing how many people don't realise!)
Anyway, returning to the original questions, a season ticket month is exactly that: a month. If you buy a monthly season ticket to start on 2nd March, it will display as "VALID UNTIL 1 APR" - so you will need to buy a new one to start on April 2nd. As mentioned, annual season tickets (12-monthly) can have extra perks thrown in. (SWT will give you a "gold card" that also functions as a Railcard when you want to buy a ticket which your season doesn't cover.)
Seasons also give you unlimited travel between the two points by any route that is allowed, for the duration of the ticket - so, in your case, you can go to and from any station along the mainline between Wimbledon and Weybridge practically for free. If you need to go further, speak to the ticket clerk and ask to buy an extension - you'll only need to pay extra for the part of the journey your ticket doesn't cover. (If you wanted to go to Woking, for example, you would only need to buy a ticket from Weybridge onwards.)
Four other points:
- Do a dry run first, before you start in your new job, at roughly the same time as you expect to be commuting. This way, you'll familiarise yourself with the route - it's better to get lost and learn then than to be late on your first day at work!
- Always use National Rail Enquiries (nationalrail.co.uk, not .com) or the relevant train company's Web site. Other sites charge booking fees.
- Get a passport-sized photograph. You will need this for your photocard, which will tie your season ticket to your identity. (The ticket clerk will also take your postcode, house/flat number and name, so if your season ticket gets lost and found, or the TOC needs to get in touch, they can contact you.)
- Don't be daunted. Commuting is easy, even fun (!) once you get into the swing of it. Given enough time, you'll know the train times by heart and be able to build flexibility into your schedule. As long as you mind the gap, stand behind the yellow line and don't be a nuisance to your fellow passengers, you'll be fine.
Good luck!
