Here's another way to think about beginning to write a play. Most plays are structured either around Charcter or Event.
A Charcter-structured play centers on the protagonists desire for something. In Hamlet, it's Hamlet' need to avenge his father's death. In A Streetcar Named Desire, it's Blanche's need to secure a safe haven for herself. In both cases, there are a lot of things that get in the way of each protagonist. That's what maes the plays exciting. We want what they want, so we root for them. it's another reason the "teenager tries to score a date" plays don't work. An audience needs a really good, really compelling reason to root for the protagonists' desire to score love.
An Event-structured play asks the characters to react and interact in response to something that is happening: a courtroom trial, a wedding, an assembly, a building project, a funeral, a fire, etc. It's not that characters in this type of structure don't want anything; but their objectives are typically tied directly to the event itself.
I mention this not because I think the best way to start writing a play is to declare " I want to write a Character driven piece!" but it's another way to potentially knock lose some idea or impulse that might get you excited. Often, an event-structured piece can help a beginning playwright work through the structure and order of scenes.
There are other structures, of course. Consider one I call the Gimmick structure. These are all the rage in a certain sub-basement of shameful "Playz for Teenz." Plays with titles like: "15 Reasons Not to be in a Play," "Ten Ways to Ruin Your College Interview," "Check, Please," and "Order Up". These plays were borne of structure, or gimmick. What you remember from them is the List format, or the rotating round-robin of dates, or the fact that Acting 1 needed a play they could memorize in 8 days.
Is it possible to write a good gimmick play? Yep. I think "Hard Candy" which kicked off the Teen Playz craze is actually a pretty solid gimmick--but it works a lot better in Schnitzler's La Ronde, or even Ives' Seven Menus. Speaking of David Ives, you could call a lot of his plays gimmick plays: a couple is fighting while their TV tries to mind control them. A bell rings every time a date screws up and it starts over again. Three moneys locking in a room are given typewriters to see if they'll type Hamlet. Two peasants try to move a heavy rock and build a tower to God. Of course, the gimmick is quick and clear, but in Ives case, that's not all that's going on. In the Teenz playz case, the gimmick is pretty much all there is. But Ives is obsessed with communication and connection and how language leads us to and away from each other.
With the exception of the Teenz Playz writers, playwrights are typically driven by a need to say something. so another way to approach your thoughts about what to write a play about is to think about what is really important to you. If you were going to gather a roomful of people not related to you, and force them to sit in the dark for 15-40 minutes and pay attention to you, what do you want to tell them? What do you think they need to hear?
Helpless Doorknobs is an incredibly flawed play as I've written it. I was more than a little hamstrung by the need to have so many characters, and to make sure that no one had a totally thankless role, but mostly, I was trying to work a full time job and deal with several crises unrelated to all the crises already going on at school. I may not have told you that my family was watching my uncle slowly die of pancreatic cancer, my 105 year old grandmother was having sudden weeks where we thought it was the end, and I was in a staggering mount of pain for my back injury but trying to hide that fact because my pain was nowhere near as important as all the stuff you kids and my family were going through. I'm pretty sure that's why a play that is scattered in its nature is so extra scattered in its execution. But throughout it all, I kept coming back to the thing that motivates me as an artist; that causes me genuine peronal anguish--the disappearence of us from each other's lives, the loss of community in the sense that I knew it as a kid. Now everyone retreats behind phone screens, refusing pretty much to engage in anything that isn't specifically targeted at them. I think it's heart-breaking, since so many of the best moments from my life have come when I was forced to engage something I was reluctant to. I'm also a long-time E.M. Forster fan, and his big theme was "only connect" and it's that connection we can make with each other--especially when on the surface we don't seem to have much in common that drives me as a person and as an artist. So I mashed up a play wherein most of the characters are swirling in a sea of self-absorption, a few are trying to connect but choosing unresponsive helpers, where the two characters determined to make a difference cannot fix everything--it's a mess, it's a terrible place to live and while all the violence seems to have no consequence, that's the biggesst consequence of all--the lack of horror we feel at our own disengagement from the lives of those around us.
So that's where I tend to start. First I get an idea of a character, of a pressing need or want, or I think about a structure--I'd like to write a play around an honor offense trial, as I think that would be interesting. But then I keep asking myself that question: If I'm going to demand that people give up their time to sit in the dark and listen to me, what do I want to tell them? What kind of experience do I want them to have?