C# Class creation to manage code in seperate class files

I am looking to understand how to manage multiple class files that will work within one application / program in C#, .net.

I have created a seperate class outside of my normal program.cs class. However, I can't seem to access the properties or methods within the seperate class inside the program.cs class.

I don't want to start creating many classes within the same main class of the program.

Thanks
Greg.