This is not a situation I have personally had to deal with, but I think the following might be sensible:
1) Be honest with yourself and the court. If (despite what you have told us) there were more occasions than the two you have told us about, then it's best to own up to them. This is because the court will impose a heavier penalty if you deny something that is true: basically, if they think you are wasting their time, they will punish you for that.
2) Collect your evidence. This is not just any tickets that you still have. It could also be proof of when you bought the right ticket - even if you don't have the ticket any more. So if you have bought tickets using your credit or debit card, find the credit card/bank statement that shows how much you paid. If what you have told us is true, then you should be able to show that you paid the price for a return ticket for the whole journey that you made. If you can't do that, it would help (a little) if you could prove where the cash came from to buy tickets at the right price (did you get the money out through an ATM?) Also, try to show when you would be likely to be making the journey: you've told us that you are at college, so try and find paperwork to show when you were expected to be there.
3) Organise your evidence. You will not make friends if you turn up with a shoebox full of railway tickets. I think the ideal would be to prepare and print off a spreadsheet, going back as far as you have evidence, showing what journeys you made, when, and how you paid for your ticket. Label any tickets you still have so you know which day on your spreadsheet they are for. If there are gaps in the history, be prepared to be asked difficult questions about why you can't prove anything.
4) DaveNewcastle said something like this earlier in the thread, but I will repeat it: at least think about talking to a lawyer. That will cost money, but if you are getting confused over how things look, think how much worse it will be for the judge or the magistrates. If you do get a lawyer to represent you, they will want some warning, so get in touch with one sooner rather than later.
5) Be prepared to discuss the amount of a fine. That means having details of any income you have so bring a recent bank statement for all accounts that you have (don't forget savings accounts) and if you have any source of income, bring a payslip for any job you have, or details of how much you receive if you are on any sort of benefits.