Before you read this user manual, you should probably read two other things.
You should know how to program in Common Lisp. If you don't already know how, you should probably read a book on it.
The Unix "man page" for SBCL will tell you how to start the SBCL environment, so you can get to the classic "hello, world" level of knowledge. It's the file called sbcl.1 in the SBCL distribution. If SBCL is installed on your system, you can read a formatted copy by executing the command man sbcl.
Besides this user manual and the Unix man page, some other SBCL-specific information is available:
The SBCL home page has some general information, plus links to mailing lists devoted to SBCL, and to archives of these mailing lists.
Documentation for non-ANSI extensions for various commands is available online from the SBCL executable itself. The extensions for functions which have their own command prompts (e.g. the debugger, and inspect) are documented in text available by typing help at their command prompts. The extensions for functions which don't have their own command prompt (like trace does) are described in their documentation strings, unless your SBCL was compiled with an option not to include documentation strings, in which case the doc strings are only readable in the source code.
Some low-level information describing the programming details of the conversion from CMU CL to SBCL is available in the doc/FOR-CMUCL-DEVELOPERS file in the SBCL distribution.