Newbies' first game designs tend to range from medium, to epic kitchen-sink games. They have no idea of the amount of work such a game will take, and no ability to execute such a game. Don't put all your ideas in one game. Just pick one thing, and make a game about that.
Your first game should be something very small. Being small is not a downside. Games do not sell more as they increase in size — probably the reverse.
Start with something very simple (remember that it needs to work two-player, so you can playtest it.) By simple, I mean really simple. Medium-light (Splendor, Carcassonne, Ticket to Ride) are all way too big. Try something more the size of Love Letter. (It's sixteen cards and a few cubes.) Do not include a board. Use maybe 20 cards, and 20 tokens, and a single A4 sheet of rules, at most. Properly executing even a small game like this is a big job. It will take at least 50 revisions to complete, and many months. It will be far more interesting and challenging than you might expect, and you'll get to experience the full arc of designing a game.
In the game Coup, players have two character cards. On a
player's turn, they use one of their characters to do something very
simple. Using the Duke gains three coins. The Assassin spends three
coins to eliminate a character. The Contessa blocks this assassination.
The twist to this game is that each player's character cards are hidden,
and you can pretend to be whichever character you like, until other
players challenge you.
This is all the game is. It's a very successful game, and an ideal size of
game to design as your first game.
I've just designed a long series of medium-light weight games. The publishable ones took over 100 revisions, and 1-2 years of design each.
As I write this, I'm just starting design on my 21st game. It's my first medium-weight game (comparable to Catan.) It is not an easy task. I can't get my head around the whole thing. Everything interlinks into everything else, and I need to work on only parts of it at a time, so I don't change too much and disrupt the larger structure. There's a lot of work in just creating the components so many times. The game will likely take 200 revisions, and up to 2 years. This is despite me working on it very regularly, and my improved design and playtesting methodologies. Update: I've abandoned that project.
Don't think "Hey, I can do it."
Design a very small game as your first game.
If you've started designing something medium or heavier as your first game, stop working on it. It will not succeed. Not only will it not succeed, it will almost-certainly not be finished. I've seen these projects go back and forth (and sideways) in endless design, and sometimes be reincarnated multiple times. Many of them have expensive, professional art prematurely done. The faster you abandon this project, the better. It's an important and painful learning experience. Hopefully, it doesn't chew up all your formative years of game design.
At the very least, start working on a new, much smaller project as well as the monstrosity.
If you're completely new, a great place to start is by inspiration — making your own version of a game you like. You know this game works, so it's possible to make another game in this space. Also, whenever you're lost, you can just refer back to the original, and see how they did it. You can even do this by just starting with the another game, and modifying it until it becomes completely different, or you learn enough to make your own game.
If you're inspired by another game, you should probably not just copy its theme as well. Choose something different.