Here are some general guidelines for finding cheap fares:
1. Book a reasonable time in advance. (With popular routes or times, that might mean up to a month or so but you can sometimes still get reasonable fares a few days ahead).
2. Travel off-peak. (e.g. midweek around lunchtime often has cheap fares).
3. Even if you're making a return journey, check out the single fares. (Two advance purchase single fares are often much cheaper than one return fare).
4. If your journey involves using the services of more than one train operator, you can often get a better deal on advance purchase tickets by buying separate tickets from each operator rather than buying a composite ticket for the full journey.
For example, I've just selected a random midweek date (Wednesday 13 September) and checked the fare currently being offered for travel from Edinburgh to York, departing at 1300 or 1400. (2 hours 30 minutes journey time). GNER have their 'Standard Advance 1' fare available, which is just �11.
If I wanted to travel to York, via Edinburgh, from a station served by ScotRail, it would probably be best to purchase separate tickets for the ScotRail and GNER jouneys. Similarly, if the journey was to Harrogate, it would probably be best to buy a separate ticket for the York to Harrogate section.
To check available fares, go to
http://www.nationalrail.co.uk/
Enter your journey details to find the times. Then click 'Get Fare Options'. If you only entered a one-way journey, the single fares are automatically shown but, if you entered a return journey, you have to click 'See single fares' as well.
If you're still confused, just post again, stating the stations you'll be travelling from and to (and preferably with details of any restriction of dates and times you can travel).
C