Last time I just kicked in the doors of a thread, so let me start with the introduction. I'm 19 years old, last year high school student, a gamer since a young age (can't say, was I like 5 years old?) and a programmer since a year ago, mostly learnt and worked with C# and Windows Phone 7, though I've already learn some C++. I've already worked on a few mobile applications and also a game before for a customer (ThinkInvisible for Windows Phone), however I want to design and create my own game - I guess you have to start somewhere.
I want to create a simple, yet entertaining and interactive game that may be even a bit unique - well I know, this is a kinda hard task. I can also create simple graphics myself, both 2D and 3D, as long as it doesn't need a graphics tablet. I feel that I'm already over the level of a simple pong, but not where I can make cost-effective pathfinding, or a good AI. Physics seem not to be much of a problem as long as I don't have to rely on a 3D engine for collision detection. I've already done some experimenting with LAN and UDP as well successfully, and apart from taking a tad lot of time to create the system that synchronizes the devices, it seems easy to use.
From trials and my own works I already know how serious a problem it is to create a game - many times even some ideas that sound simple can become extremely annoying and demanding in a short timeframe. So I'm here to ask you: how should I start designing a game (for now for mobile phones, since I don't thing simple games are fit for PCs) without going overboard with resources or advanced, high-demanding algorithms, like pathfinding, which appearently eats away a mobile phone easily, and such?