Before you even think about turning your computer on, you need to design your game. It's not enough to have a rough idea in mind. You need to write down all your ideas before hand to start working. The first phase of game design is brainstorming. There are different techniques and methodologies of brainstorming that, unfortunately, we don't have the time, in this small section, to describe. However, the final outcome should be tons of paper with thousands of ideas written down. It's not meant to be a work of art, but the foundations upon which your game will be built.
Note
Some information about brainstorming can be found also in a practical book, Gamification with Unity, by Packt publishing. You can find it here: https://www.packtpub.com/game-development/gamification-unity-5x.
The next step is to refine your ideas, discard (or keep for other projects) the ones that you don't need, and organize them in a coherent form.
The final result should be something like the following.
Panda Invasion is a 2D tower defense game. In fact, hungry pandas are invading to steal all the sugar from the player. He or she has to push back the pandas by using cupcake towers. There are different kinds of cupcake towers that the player can decide to place in the map. In every level, there will be a path that pandas will follow. Furthermore, they are spawned at the beginning of this path. At the end, there is the ambitious sugar castle that the player has to defend. If the pandas steal too much, bringing the sugar-meter to zero, the player has failed his or her important mission. On the contrary, if he or she is able to push all of them back, the player will gain victory. However, cupcake towers are not free. In fact, the player has to buy them by using candy. Every time that a panda is pushed back, then the player will earn a certain amount of candy. Furthermore, the player can use candy to upgrade the cupcake towers and make them even stronger!
From this excerpt, you are now able to understand what we are going to do in this book. You also have a basic idea of how to write down your ideas. I strongly suggest that you always do this step, even when you are the only developer, and especially when you have a team.