Something small and simple, so you could try your hands at a text-based adventure. The advantage: you can make a complete (albeit small) game with very simple tools. For example, make a game where your goal is to kill the dragon. First you need to explore a cave to find gold, then you have to buy a sword from the blacksmith, and then you have to train at the training ground to be able to slay the beast.
After you've got this, you already have a complete game to be proud of! In the meantime I'm sure you'll have had tons of ideas that would be really cool. Maybe you want the fight itself to be fleshed out with attack, defend, and skill commands? Maybe you want to be able to buy different weapons at the blacksmith, even if that doesn't affect the rest of the game? Maybe you want to change it so the dragon flees before you kill it to set the player up for another adventure (one that's slightly bigger and a little bit more of a challenge for you to make)?
Wait with implementing these ideas until you have your 'first draft' completed, and then work on adding/changing stuff one at a time so you're not overwhelming yourself. Trust me, that's the #1 pitfall for your first project. Check out the first few videos in this playlist for some great tips on what pitfalls to look out for: https://www.youtube.com/playlist?list=PLhyKYa0YJ_5C6QC36h5eApOyXtx98ehGi