<p>In a progression of languages, I have been learning C and C++.  Now I would like to learn C#.  I know there are some drastic differences between them - such as the removal of pointers and garbage collection.  However, I don't know many of the differences between the two.</p>

<p>What are the major differences that a C++ programmer would need to know when moving to C#?  (For example, what can I use instead of STL, syntactic differences between them, or anything else that might be considered important.)</p>

C# for C++ Developers
is a great place to start.  It is a table that lists the most important comparisons between the two languages.
Once you have explored some of these differences, you might choose a self-contained project you have written in the past in C++, and re-write it in C#.  In your first pass, you will probably just end up translating directly across, using the same design and algorithms.  As you become more comfortable with C#, you will recognize ways to take advantage of language features only available in C#, as well as the incredibly versatile .NET Framework.