cat yourfile.txt | tr -dc '[:alnum:]\n\r' | tr '[:upper:]' '[:lower:]'
The first tr deletes special characters. d means delete, c means complement (invert the character set). So, -dc means delete all characters except those specified. The \n and \r are included to preserve linux or windows style newlines, which I assume you want.
The second one translates uppercase characters to lowercase.