If all your travel is simple there and back stuff, then it is time to look at the online booking tools. Before we do, however, let's bear another point in mind. Long ago, before it was decided that everyone had a divine right to fly everywhere and anywhere for very little money, simple stuff was just that, simple.
At best, for the "Monday to Friday" business person, you got what was termed a "Eurobudget" ticket which meant you took your flights and if you needed to change things you paid your (what was then) £20 and boarded the next plane home. Irrespective, incidentally, of whose name was written on the side of the plane. You could get a seat reservation, no one minded about a hold bag and even light refreshment during the journey.
Nowadays, the story is radically different. There is a whole selection fares, fares to be wary of and some simply daft fares on many short haul (i.e up to two hours-ish) routes. Here we come into a new subject, "cheapest" can be a very expensive way to book travel. "Best value" is a totally different story!
The fare rules must be read. This has come about with what airlines call "unbundling". That is, taking bits of the service that used to be included, saying that you (apparently) wanted "options" and then making you pay extra for things such as seat reservations, hold baggage - and most importantly - flexibility.
Some of the cheapest fares (and in the case of a few airlines, some quite expensive fares) do not allow any changes at all, under any circumstances and one must be very careful that your ticket does not fall into that category (unless you are 110% sure nothing will change, of course). The difference between a fare where you can make no changes and one where you can, I have seen for as little as £8 - and that on a £300 fare!
Of course, if you are using an online booking tool, which restricts you to buying "the cheapest" then that simple flight can become a very expensive affair.