Oftentimes when I read articles on game design they are about what the first time designer should do. However, this go around I would like to emphasize what a new designer ought NOT to do.
I was reading an amazing business book recently, Good to Great by Jim Collins, and it mentioned an interesting insight - that companies need to focus not on a to-do list, but a to-don't list. Time is our most precious and finite resource. We need not waste it on frivolous activities that don't provide the largest possibly return on our investment (our time). Designing a game for the first time is tough and will take way longer than need be if you focus on things that should not matter as much in early iterations of your first design.
- Don't design a game you won't enjoy playing
- Don't worry about balance right now
- Don't get too attached to your game
- Don't worry about it potentially being too similar to something else
- Don't stop
Don't design a game you won't enjoy playing
Eventually when you create games regularly, not all games you create will be 100% up your alley, but for your first game you should design something that you want to play. You're likely going to be doing the majority of the play testing and you need to really enjoy it. You need to know what you like about it and what is good so if it is a theme or uses mechanics similar to what you play you're likely going to have an easier time designing it.
When play testing your game you're going to need it to appeal to some small subset of the market, but you don't have to worry about commercial value yet. When you get the game complete you might decide it is not good enough to self-publish or shop a publisher for. Most people's first game does not go anywhere - which is fine. If it is good enough your weird little theme will likely change and if any mechanics don't fit the mass-market you can always design a similar second game and keep your first as a game you can play with your friends.
Don't Worry about balance right now
So many designers I meet want to create a game that will be perfect right off the bat, but I have a little secret - that doesn't happen. It is perfectly acceptable not to want strangers to play test a game that has not been thoroughly conceptualized. However, if that is the case what you'll want to do first is fully prototype the game and play with yourself or friends/family first.
Try to make the game fun. Playtesting will allow you to fix balance issues, but most people will play a game that is not fun. If it is enjoyable, albeit unbalanced, that is fine for now.
Don't get too attached to your game
The brilliant thing about play testing is that people will find the errors in your game. All those little mistakes, the miscalculations and balance issues you didn't worry about earlier are going to show through. You'll likely get more negative feedback than you would like, which is why you should treat the game as if you have spent no time, effort or love on it.
We know it is your baby, but if you treat it that way you're going to have difficulty making the changes necessary to make the game better. You cannot get too content with where the game is. Eventually there is going to be a time when you think the game is finished and you will then have to send it to play testers with the rules and without your guidance (blind play testing). They will likely find a few issues with the rulebook and maybe even a few minor rules changes. Then hopefully your game will be complete and it will be better than it ever would have been without the extensive testing.
Don't worry about it being too similar to something else
My brother recently started designing games and the reason it has taken him so long is he kept starting projects only to quit when he found something similar. Unless you're intentionally trying to rip off another game it will likely not be too similar. Either way as your first game it doesn't really matter. What you need to learn is the process it takes to design a game from start-to-finish. That is much more important than anyone accusing you of copying Munchkin.
Once you're doing blind testing it is likely someone will let you know if it is an exact replica of XYZ. If it is find ways to make the game better, and likely it will end up very different from the other game. If you want to keep it close to its current design iteration then play the other game and see if you have any issues with how similar (or not) it is. Maybe you find that the similarities are sparse.
Likely the most important step is not stopping. You need to finish the process. Complete the game, even if it is something you don't like and are not proud of. As a writer I could never finish a book. Every time I started writing after about 10k words I would learn to dislike the story I developed and want to start something else - don't do that. Likely just like I had writers block, you'll have a block with your game.
Sometimes it is a good idea to take a break and work on something else while you try to figure out how to fix a certain problem. However, with your first game it is probably a better idea to just keep working on it until it is "finished." Once it is good enough then maybe you can table it until you have better ideas. Though, be weary of just half-completing all projects.
At some point of time your goal will likely be to have one of your games in print either through self-publishing or through another publisher and that won't happen if you don't finish your games.