3 Tier Application Design

I am designing a windows application where I would like to make it a 3 tier
design.
The thing I dont know is even if I make a dll with API's that will be used
by my UI tier. How will the UI communicate with the API's residing in the
DLL.
As you now know I am very new to this and have no clue how will my UI will
call functions in my DLL which I plan to deploy on an application server.

Basically I have a UI which I will distribute to the clients and install the
business logic over a central location and my database residing on a
central location too.

Now regarding the dll which I would like to design. What steps I should take
so that if I need to change few functions in the DLL it wont break the code.
I mean should I always use Interfaces and then implement my functions or
something else.

Anybody who can even suggest me a book or an article which describes a 3
tier design used in .NET windows application design.

Any help in this regard will be helpful.

Thank you,

Abhi