When you begin writing a mod you should start small. Don't plan to write a Total Conversion (TC) from the very start. If you set goals that are too hard to reach, you'll get frustrated in the process of working towards them. It is much better to set a series of small goals and work to each one in turn. Start with a simple idea that could be expanded into a larger game. Always work on small, managable chunks that could each be released in their own right. If you do undertake a large project, organize your features into a release schedule. If your game is going to have 5 new weapons, make a release with 3 while your work on the others. Pace yourself and think about the long term.
Everyone is an idea man. Everybody thinks they have a revolutionary new game concept that no one else has ever thought of. Having cool ideas will rarely get you anywhere in the games industry. You have to be able to implement your ideas or provide some useful skill. This also applies to mod authoring. If you become a skilled or notable mod author, you will find people propositioning you to implement their ideas. Never join a project whose idea man or leader has no obvious development skills. Never join a project that only has a web designer. You have your own ideas. Focus on them carefully and in small chunks and you will be able to develop cool projects.
Remember that developing a mod doesn't mean much if you never release it. Scale your task list so that you can release something quickly, adding and improving features as your mod matures. If you hold back your mod until everything is perfect, you'll find yourself never releasing anything.
Now that you have your idea, you need to choose what kind of Unreal Tournament mod type is right for you. There are basically three types of mods. We'll go over each one in brief and then focus on them all individually.