It's very rare that airlines even offer round trip tickets these days. I remember when they did and you could get things like APEX tickets at a big discount, but that has all gone and the normal thing is you book a flight to go there, another one to come back, and you pay for those two single fares all at once.
So that's what you do - pick a flight to go, another one to come back, and what you pay is for those two added together.
Just out of interest, book online and you don't even get a ticket. When you check in at the airport (or online the day before if the airline offers this - most do), you get a boarding pass and that gets you the rest of the way on to the plane, So they only want to see your ticket at check-in, but now they all have computer terminals at check-in and can look up that you've booked and paid on that, do we need tickets at all? No we don't. So the average big airline stopped doing it years ago and saved a lot of money on paper and printing.
So all you actually get is a confirmation email and I print that out so I can't forget any of the details.