It’s a pretty relaxed, ~2 week jam, you’ll make it! 
Few tips from me:
- don’t overthink the idea, just go ahead and select any initial thought you had quickly - I usually have an idea for a game I want to create before game jams - then I just try to make it fit the theme, but sometimes indeed, I change the idea when the theme is announced, but you’ll have to do it fast, in the 1. day
- make something that has a quick to achieve core gameplay loop, avoid making complex genres
- you can re-use existing ideas, add a twist eventually, or try a hybrid
- prepare assets, simple sounds, basic graphics, especially GUI, upfront, for the game genre/setting you’d like to do - before the jam - then fit to the theme with the mechanics, not the assets
- even prepare a starting project, game jam period is for the gameplay creating and testing!
- prepare a simple main menu structure before the jam, passing between screens - it might be helpful to add a few pages of written tutorial quickly or introduce story then in the beginning, before opening the game itself, then you can adjust to the game
- complete the core loop at least 1-3 days before the jam and spend the last on testing and polishing
- make the website/project on itch before the jam, you don’t have to publish it until you’re ready - but upload the builds there to test it’s working in browser! (If you’ll only make a desktop build, chances that others will check out your game drops drastically), at the end - you’ll upload the final build and update the description of the page
@Asatte make a great video on building and uploading on itch:
@benjames171 is our game jam veteran and released a lot of the Defold projects on Github:
Those are a great base, especially for beginners to learn how to make small games focused on gameplay.
@britzl also have a repository of many, many examples: