There is a link to manuals at the top of the page. The CICS Language Reference manual in Appendix A, chapter 9 talks about the BMS macros and what can be specified and how. I recommend you spend some time in this manual to answer your questions about coding up the mapset, map, and field definitions.
Are you moving LOW-VALUES to the output map or the individual fields in the map? IIRC, moving LOW-VALUES to the individual fields will override the INITIAL attribute -- but it's been a while since I've done a map so don't quote me on that.