@3lovingmom it is fine for now to put “undecided”. My DS submitted today as “undecided”. When he did, he got a confirmation from NMSC, and in the confirmation email it has a link to change/update first choice college when he is ready to do so.
You can read the attached for more color. If you find out you are a finalist in February, it looks like it may be a good idea to have identified your first choice and to make sure NM has been notified and update their records before March 1. This apparently is the date they will begin referring finalists to the colleges for scholarship consideration by colleges that are sponsoring NM scholarships. You will then know you are in the “first group” referred to your “first choice” school.
http://nationalmerit.imodules.com/s/1758/images/gid2/editor_documents/merit_r_i_leaflet.pdf
Someone who has been through this please correct if I am misunderstanding the process. My first rodeo.