A good requirements document would help you not be so overwhelmed. Use cases are a great way to figure out the functional requirements.
Sample system requirements document - AC
is an example. The main thing to notice is You have a perfect case scenario. Then after that you think of any alternate paths that may take place and document them.
Then you want to model out a class diagram. Visual Studios has a class diagram layout you can use. It can generate diagrams on code you already have.
If you want to get real fancy you can use UML and use something like MS Visio.
UML basics: The class diagram has a nice little starter for UML. UML is something that is used a lot in real world OO software development and is something you will want to get familiar with at some point.
In short, you want to model out all your methods, data members and events(C# terminology) before you start any actual coding. Before you do that you want some requirements documents so you know what you will need to accomplish.