It might be surprising and counter-intuitive, but I think one should avoid training plans and schedules the first few years after deciding to run competitively. Run when you want, run as fast as you want, run as far as you want. Race frequently.
Runners improve dramatically and inconsistently the first three or four years they run, regardless of how they train, as long as they regularly run and race. The most common mistake made during this time is following a plan and then attributing one's success to the plan, rather than to improvement through experience. The second most common mistake is following a plan until one reaches a plateau in performance, switching to a different plan, and then attributing any improvement to the switch in plans. There are hundreds, if not thousands, of coaches whose reputations are built entirely on this phenomenon. It is a mistake to think that one will improve faster or more consistently if one follows a plan during this time; improvement will come at its own rate.
One reason not to have a schedule in these early years is that this is the time to develop a fondness for the sport, which is jeapordized by being told what to do and how to do it, every single day. Most runners will reach a point where they're tired and need a day off, but their schedule has them doing a tough run, so they either do the workout and then feel even more tired for the next workout (eventually leading to burnout), or they fail to do the workout, either falling short or skipping it altogether, and, if they don't reach their goals, they think it's because they didn't follow the schedule closely enough.
The most important thing to do in the first 3 or 4 years of training is to try a variety of races. Run races on roads, tracks, cross-country and trails; run a wide variety of distances. Over this time, one will discover what one's strengths and weaknesses are. As a general rule, what one does best, what one enjoys most and what one does most frequently will turn out to be the same thing. Race often during this time, as long as you want to race and you enjoy it; this is when you'll get "the most bang for your buck." Later, when improvement has slowed and one has to train more and harder just to shave off a few seconds, one will need to race less frequently, but this is the time to see what you can do with a minimum of effort.
With few exceptions, I don't recommend racing marathons in these first years (I ran a marathon the week I turned 16 years old, but shouldn't have; on the other hand, I trained with a runner who ran a 2:28 at age 17 and another who ran 2:16 at age 19, they being exceptions). It is difficult to run a marathon well without considerable planning, though with consistent training it is not difficult to finish one. If you want to run a marathon during this time, don't aim for more than finishing.
The other important thing to do during this time is to write down what one does, but not refer back to it. Record keeping is boring, but it is an important habit to develop. Later, when one is following a plan, it is often informative to take a look at what one did during the early years - after one is certain that it's imperative to do a certain workout to succeed, a look back will often show that it is not. At a minimum, write down how far you ran and how long it took; for most runners, measuring distances to an accuracy of a half-mile or mile (or 1 km) is sufficient and, if you aren't sure of the distance, make a guess. I didn't start writing down what I did until I'd been running for years and then I had one entire year written on one sheet of paper, which I still look at, wondering what pace my long runs were and what "fast" meant. The reason I say "one should not refer back to what one writes down" during this early time is that it often leads competitive runners to compete with themselves, to feel that they have to continuously strive to do more, faster, rather than to let the body adapt at its own rate.
When one is certain that one is no longer improving and it isn't just a temporary plateau in performance, it's time to start planning. That is what the rest of this series will entail.