First step with game jams that have the "24-hour" after deadline bug fix rule - submit early! Once you have a few game jam entries you learn to upload outside of expected busy times. I uploaded almost a day early and there was a lot of bugs and gameplay feel missing. (Just note that its an early build in the description)
For a 7 day game jam you have the time to spend the entire 1st day to plan, think about design and scope the project. Try to get to the core gameplay loop and cut what feels like "extra fluff". Think about games with 2-3 mechanics AT MOST and polish those well. Figure out the hard parts of the project and tackle those first as they will likely take the most time.
My first day was thinking about design using Obsidian, getting references and doing some technical tests with HTML5. It turned out HTML5 wasn't going to work with multiplayer so I had to scrap that whole part of it. Good thing was I did that on day 1 and not day 5. So on Day 2 I was starting a fresh project with no real development done yet but a "relatively" good plan.