For planning a game, it can be a very good idea to write a pitch with a few basic questions answered. The order people do things in can vary a lot. Temmie drew her game first. Toby Fox and Zunpet score a game before they make it. Some prototype rapidly-but once you have a solid idea what the game will be about, you have to ask yourself some questions to make things easier on yourself:
Who is the main character?
What is the genre and inspiration?
What kind of world do you want to design? What does it look like or contain?
How do your characters move (if they do)? Or how the player interacts and the stats to use if you're using a visual novel or text game format.
What art/music style do you think would work with this?
What's something special you have to note about the game?
Does it have NPCs? How do they work? What do they do?
You don't have to come up with a title right away! You can always come back to a question when you figure it out! Try to be specific as possible. When thinking of how to do this, think of what's the minimum amount that you'd be able to accomplish-it's best to have what you need to do done and have time to put in a little extra than to run out of time on features you didn't know how to implement.
And don't worry if you're running late. As long as you start and have something-that's still a success.
Here's a little example:
Working Title: Feather Maiden
Lead Character: Female human shape-shifter; use of a sub-item/power causes a change in form.
Genre: Cute platformer, inspired by Hudson Soft and Sunsoft platformers. with Metroidvania elements similar to Wonder Boy in Monster Land.
World design: Three main areas, with two sub-areas. Five areas in all. (Forest, field, river), (Barn, village)
scheme: (Keyboard/Controller) Walk, run, jump, glide, duck,
close-range ground attack, air glide attack#, protective block#, wall
kick/grab#, powered charge ranged attack#, long-fall protection#.
Hidden items: Health upgrades and items for all moves marked with #. Each area has one upgrade item via a boss who demonstrates how to use it. The non-special upgrades are either hidden or require some minor puzzle solving.
NPCs: There’s a few places to go inside and get requests from NPCs who have on offer feathers, the most important item for the character to find. Some are very simple, some are not! They are very simple, broadstrokes NPCs. They should be largely cheerful or comical.