There are countless ways you can stumble upon an interesting game idea, but finding that spark of inspiration can be harder than it sounds. The best advice I get from game designers is to build game ideas around real-life actions or activities. So instead of starting by imagining how many people can play your game, whether it involves a board or cards, or how you'll score, start with an actionable, real world narrative that you'll do yourself a favor. Doing this can help you figure out what you want to achieve with your game and allow the game's mechanics to blossom more naturally.
For example, let's say you choose gardening as an activity anchoring your game. What considerations are relevant to this activity? Do you set up your garden in a series of planters? If so, maybe every player should have a board with limited space to grow things. Perhaps some plants will be larger than others, and you can assign their point value to motivate players to think strategically before placing them in the garden. In real life, planting certain types of plants next to each other can complement or hinder their growth. This works as a conditional scoring mechanism where you can gain or lose points for placing specific plants next to other plants.
It's also important to consider how players feel while playing the game and how those feelings relate to the overall narrative of the game. For example, many people spend time gardening to relax and unwind, or spending time with other people and growing food for their community. For this reason, maybe you avoid using timers in your game to avoid stressing the player. Or maybe you want to make the game cooperative to promote a cooperative atmosphere. Possibilities are only limited by what you can come up with, this activity-based approach to design allows you to stay intuitive and creative with game mechanics, even as you're designing your first game.