Well, there is no simple tutorial to multithreading. First because multithreading is never easy. Second, because there are many ways to do it. Third, each version of .NET has new ways of threading and parallel programming. So dependent on what version of .NET you use you will have to use other techniques.
Personally I liked Sacha's tutorials on multithreading and TPL (Task Parallel Library, which isn't multithreading per se).
And dependent on how deep you want to get into this stuff there are simply some .NET Classes and libraries you can use that make life easier, you can try to really understand what's happening and try more advanced threading, and you can even create your own Classes or framework and apply all sorts of design patterns that work especially well with parallel or multithreaded environments.
So here are some articles that I found especially useful.
Threading:
Beginners Guide to Threading in .NET: Part 1 of n[
^]
Beginner's Guide to Threading in .NET: Part 2 of n[
^]
Beginner's Guide to Threading in .NET: Part 3 of n[
^]
Beginners Guide to Threading in .NET: Part 4 of n[
^]
Beginners Guide to Threading in .NET: Part 5 of n[
^]
TPL:
Task Parallel Library: 1 of n[
^]
Task Parallel Library: 2 of n[
^]
Task Parallel Library: 3 of n[
^]
Task Parallel Library: 4 of n[
^]
Task Parallel Library: 5 of n[
^]
Task Parallel Library: 6 of n[
^]
Then there is stuff such as PLINQ:
Introduction to PLINQ[
^]
Parallel LINQ (PLINQ)[
^]
Reactive Extensions:
Reactive Extensions[
^]
Fun with Rx[
^]
Some other articles:
Multithreading Demystified[
^]
Introduction to Parallelism in .NET 4.0[
^]
Introducing .NET 4.0 Parallel Programming[
^]
Asynchronous Method Invocation[
^]
Patterns of Parallel Programming[
^]
.NET asynchrony in the UI context[
^]
System.Collections.Concurrent Namespace[
^]
Well, I think I've given you enough articles to start from.
Good luck!