Pica Congrats on deciding to make a game - you have a fun ride ahead of you, and C2 is a great tool to use for realising that goal.
My recommendation for your first game would be to start small. Very small. Leaping into a full-blown platformer or RPG could well be a recipe for misery because at this stage there is so much you don't know and the learning curve is steep, which may put you off completing your project (that's not a personal criticism - I've been at this gig for many years now and there's still plenty for me to learn - I still bite off more than I can chew from time-to-time).
It's better to familiarise yourself with the whole process of developing a game, from initial concept through to release, and repeat this over a number of small projects to fully grasp your capabilities and interests than to plough into an epic project that could take years to come to fruition.
Worry less about the story/theme at this stage and focus more on finding some core mechanics that work well together and can be used to produce a variety of interesting interactions.
Go through the example projects that come bundled with C2, reading the code and understanding how they operate. Play around with them, adjusting variables and adding in new objects and events to get a better idea of what can be achieved with that template. Most likely one of these will strike you as a suitable starting place for your own project. If you can't find anything that suits make a few prototypes/experiments to test out any concepts that you have. Try out your prototypes on friends and listen to their feedback.
Read the manual, search the tutorials and forums to find answers to any problems that crop up; most of the issues you initially face will have been already been asked and answered on this site. This page by 
Kyatric, one of our benevolent moderators, is always a good place to start:
Once you've found a core concept that you're happy with, set yourself strict limits on what you want to make - for example if you're making an action puzzle game restrict yourself to ten small levels, maybe even less. Write down a task list at the start and update this as you progress. Record how long it takes you to complete each task - this will give you a better idea how long it take you to complete the entire project, and helps you set achievable goals both on your first project and subsequent ones.
Have a go at producing the art and audio yourself, it's good to familiarise for yourself with the tools and pipelines for developing the assets; try not to get too caught up with the visual presentation at the start of development - get the gameplay working nicely first and then let that guide the art style.
Keep iterating and testing throughout the project, with friends and/or post what you have on the forum in the Works in Progress section; listen to the feedback even if it hurts - better to squash/change a bad idea sooner rather than later.
Hope that's useful to you!