You're probably better off learning C on whatever computer you now use. This way, you're only learning one thing at a time instead of several things all at once. Less frustrating in my opinion.
Get a good book on C. I always liked the Kernighan & Ritchie book "The C Programming Language". You should always have a book to use as a reference.
Embedded systems can be strange. Some of the smaller processors/controllers use non-ANSI extensions to C in order to make access to their features more efficient/possible. There are things you have to do to make C work on an embedded system that have already been done to work on a PC/Mac/Workstation, unless you've bought a C that has already been tailored to work for the particular CPU you are using (like Zilog's eZ8 eval system).
One book I thought was helpful for embedded systems was "Programming Emebedded Systems with C/C++" from O'Reilly. They also have "Designing Embedded Hardware" if you're new to hardware too.