You might want to check out some of the books (see left side of screen). There are some that cover 8051 and C/assembler. Even if you know C, the embedded environment is a LOT different than running Borland C on a PC. (Especially with a command line toolchain like SDCC.) There is no operating system to do stuff for you--you have to do it all yourself. And that includes the low level I/O like getchar() and putchar() that printf is probably calling (the compiler can't know what hardware you might be running on, it doesn't make any assumptions).

You might want to get a copy of Keil's demo CD, it has a lot of good 8051/C tutorial stuff on it. With the basics down, then SDCC would be more useable for projects bigger than Keil's demo will allow. (Of course, once hooked on Keil you'll want to get the full version!)