I really need the cheapest train fare poss?

me and my boyfriend are going to stay with my dad for about 4 days next week, from tuesday to saturday/sunday, but my dads really busy with work all week so i need to get there on my own money but i didnt know this untill today after i got back from a nice big shopping trip, ironic right. so i need the cheapest train/coach fares available from halifax/leeds area to dundee for the 18th or august, coming back isnt a problem its just getting there..... im 16 and my boyfs 17, i have a half fare travel card thing for college so i get half far travel, but he doesnt + he's also pretty much spent up for the next 2 weeks..... any ideas? :)