Do NOT (I repeat NOT. Got it? That's NOT!) use third-party sites such as Trainline.com. They can't find fares which aren't on the National Rail website, they often charge MORE (because they add on a booking fee) and it's not unknown for them to issue tickets which aren't even valid for travel at all. (When I ran a railway station I got fed up of explaining to customers that they couldn't use their tickets, purchased via Trainline.com, because they had been offered using a promotion from company A, on a train from company B,with seat reservations on a service which didn't even have numbered seats!).
The National Rail website (as per Jackdaw33's link) is always the right place to start. However you might then want to try 'ticket splitting' where, instead of purchasing a ticket from A to C, you buy separate tickets from A to B and then from B to C. Sites such as this one
might be able to help
http://splitticketing.co.uk/
but they're far from perfect. In particular, they generally only work where there are savings to be made through buying two separate tickets for the journey and completely ignore what can be done by buying three (or more) tickets. (That can make a big difference).
So, if you can identify one or more possible places along your route where it might be possible to split your journey, it's often best to do the searching yourself (using the National Rail website).
Alternatively, post your travel requirements on AB and let one of our rail experts do the work for you. (Baza and I have both run railway stations, and know how the ticketing system works. Dzug2 and Mushroom25 are also experts in the field).
For example, Jubieanna asked about return travel between Chepstow and Gleneagles here:
http://www.theanswerbank.co.uk/Travel/Question1511868.html
The normal off-peak rail fare is £171.20. A ticket-splitting website could only get that down to £167.33, saving less than £4. By doing a manual search (and splitting the journey into three sections, rather than two) I got the cost down to £110.70, saving around £60.