How will I know when to create an interface?

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it solves this concrete problem:

you have a, b, c, d of 4 different types. all over your code you have something like:

a.Process();
b.Process();
c.Process();
d.Process();

why not have them implement IProcessable, and then do

List<IProcessable> list;

foreach(IProcessable p in list)
    p.Process();

this will scale much better when you add, say, 50 types of classes that all do the same thing.

Another concrete problem:

Have you ever taken a look at System.Linq.Enumerable? It defines a ton of extension methods that operate on any type that implements IEnumerable. Because anything that implements IEnumerable basically says “I support iteration in a unordered foreach-type pattern”, you can define complex behaviors (Count, Max, Where, Select, etc.) for any enumerable type.

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