Strings of ACL2 characters are written as sequences of characters delimited by ``double quotation marks'' ("). To put a double quotation mark in a string (or, any other character such as backslash or newline that seems to cause problems), escape it by preceding it with a backslash (\).
The function stringp 
 recognizes strings and char
 will fetch the nth character of a string.  There are many
other primitives for handling strings, such as string< 
 for
comparing two strings lexicographically.  We suggest you
See programming 
 where we list all of the primitive ACL2
functions.  Alternatively, see any Common Lisp language
documentation.