Hi Luka. My advice to you if it's your first Jam and you're planing to go Solo, is KEEP IT SIMPLE.
Once the Theme be announced, and you have in your mind something to do, keep it as simple as possible. Don't overcomplicate things. Don't try to make a GTA or a Skyrim cause you WILL fail to succeed in the week you have to do the game.
Also once you start, try to finish the core gameplay and mechanics in no more than 2 days. My experience in this jams is ALWAYS upload the game just a few minutes before the deadline, which is extremely stressing.
Wish you good luck and most important: Have fun!