There are three main types of tickets: Advance, Off-Peak, and Anytime.
Advance tickets' main benefits are that you can save quite a but of money over the Anytime and Off-Peak tickets. However, you are tied down to a specific train, and if you miss it due to an issue outside of the railway e.g. traffic, or over sleeping, then your ticket becomes worth nothing.
The Off-Peak return from Preston to Euston is currently priced at £95.40, but carries some restrictions itself. You must arrive into Euston before 07:20, or after 11:29. You cannot depart Euston before 09:26 or between 15:01 and 18:44. Those restrictions all apply Monday to Friday, except the last one, which only applies Monday to Thursday.
I won't go over the Anytime Return as it is ridiculously expensive.
In terms of buying the ticket, you can support your local ticket office and buy it at Preston (although I would recommend against this as they use a different type of ticket paper which is the opposite of durable), or you can buy online on any Train Operating Companies' website. Please avoid TheTrainLine as despite their claims, you don't save any more money with them, and they charge booking fees.
Tickets can be shown on your phone as an e-ticket (generally a PDF), or as an m-ticket (which requires activating before use). You can also collect your tickets at the station with your original payment card and booking reference, or have them posted out to you.
Finally, you could try a site called TrainSplit. The site splits your tickets up to save money. Despite what it sounds like though, unless you choose to do so, you don't have to get off at every stop, and you don't have to play musical chairs as it will aim to reserve you the same seat throughout.
Depending on your age/personal circumstances, it might be worth looking at the range of Railcards, which can save you up to 33% on tickets. Especially if you're planning on travelling more, it may be worth the, normally, £30p/a cost.
Hope this helps - feel free to reply if you need anything!