As with any new language, once you're comfortable with a small core of the language, start working on your own problems. Start with little made up problems, and move on to useful tools as soon as possible. You don't have to be a master to write your own tools, and you won't become a master until after you've written your own tools.
HN says that Learn C The Hard Way by Zed Shaw is good.
http://www.hnsearch.com/search#request/all&q=%22learn+c%...
http://news.ycombinator.com/item?id=1027549
As with any new language, once you're comfortable with a small core of the language, start working on your own problems. Start with little made up problems, and move on to useful tools as soon as possible. You don't have to be a master to write your own tools, and you won't become a master until after you've written your own tools.