I think it's a good idea to participate in a MAGS-competition and do a small game within a month (something I should've done a long time ago instead of starting several large projects that never got finished). Just go for something small and plan a game with a few rooms and few characters and see how much you can accomplish in time. This might help to estimate the amount of work and the time you need for something larger. I recommend playing a few MAGS or other short games to see how people design small games.
+1
If you want to get a good idea for scope and how to make games, participate in a MAGS competition.
That's my first bit of advice for making a small game.
Now if you want to know where to start, here my magic little formula.
First, come up with an idea. A premise. For example, a pirate is stranded on an island and wants to get off.
Second, list the main events in your story through a series of bullet points. For example, pirate makes plans to build raft, then builds the raft, then gets provisions, then sails off.
Third, write a list of things that need to be done to accomplish each bullet point, preferably working backwards. For example, pirate makes plans to build raft by getting paper and pencil. Pirate makes pencil by getting wood and graphite. Pirate gets wood by breaking branch off tree.
From there, it's just about making all of the necessary resources and putting it all together.
Also remember, things don't have to look good on the first pass-through. You could try to make it perfect the first time round. But I find it helps a lot if you make all of the resources quickly and without any attention to detail, as it gives you a good sense of what you need to make and what might need changing. In other words, get a playable prototype out as soon as possible. (In a larger scope game, you might want to do that in chunks. But in a small game, you should be able to the whole game.)
I hope that all helps.
