As a hobbyist programmer/game designer, I would echo the sentiments of both SchismNavigator and Chronopolize.
I love working in Unity because it is relatively intuitive but with vast flexibility and tons of online resources. Early on, I relied heavily on free resources to augment my ideas, and these can typically be tweaked. Good free resources can provide a starting point for having something functional that you can tweak and learn from.
Also, there are tons of high quality video tutorials for Unity, both for the Unity engine software as well as coding in C#. Almost anything you want to learn is available for free on YouTube.
As for how to start a project... I have found it immensely valuable to do a mind map using a tool like Coggle to flesh out my ideas and organize what kind of assets I'll need to create. For example, if I wanted to make a simple roll-a-ball game (which, I should say, Unity has a tutorial and provides assets for), I would consider the roll mechanic--does the player tilt the game board, directly move the ball, etc. Then I'd consider what kind of assets would be needed: for example, a controller script for the ball (or board), the ball and board 3D models, the textures, shaders, etc., any sound effects, etc. By brainstorming what is needed for each element of the game, it breaks the process down into manageable pieces for me. You could apply this concept in myriad ways--using note cards, an Excel spreadsheet, a doodle on a piece of paper, etc.
Everyone seems to want to make some grand project as their first game. I recommend getting your feet wet by picking one or two simple mechanics you want to play with and build a simple game. This probably means a game that focuses more on level design than complex inventory management, spells, etc. That said, even an RPG can be simple if you limit the scope of what a player can do.
One interesting resource I've found helpful for considering game mechanics and design is the 4Keys2Fun chart. If this resource piques your interest, I recommend watching one or more of the videos in which the tool's creator discusses it--it makes more sense with some level of explanation.
I'd also recommend getting involved in the game development community. I personally have found it tremendously helpful to have other designers/creators who are willing to help--and who I can help, at times. You might consider Game Dev League (that's the community I'm part of, and if you're polite, it's amazing what you can learn there).
Hope that's helpful!