How Do I Buy a Flight With Layovers?

When you book a ticket or a flight, you have to decide whether you want to buy a round trip ticket or a one way. A layover just refers to a stop in a city between your original place and your destination. So for example, if i buy a flight from san diego to london, but i have to stop in new york first, this would be considered a layover. I'm sure most of you would agree. I prefer direct flights. But sometimes there aren't any direct flights between two specific destinations, so sometimes lay overs are necessary.