problem with Lex.

Hello, I am having little issues with learning Lex. I am trying to create a programming language, but I dont know where to start as far as writing the grammar. I have looked into progams such as ANTLR, YACC, and JavaCC. But I don't really get where they are getting all the code from. I also kind of dont fully understand what writing lex does, as far as declaring a programming language's grammar. Also, by Grammar, they mean the Syntax of the programming language correct?

NOTE: I am just trying to create a simple programming language, nothing too sophisticated..