Whether you need to get through school or you need to modify a commerce application, adding columns into mySQL is something that must be done to custom tweak databases.
The following 3 commands can be used in at the mysql prompt or made with an sql query with phpmyadmin.
ALTER TABLE mytable add column newcolumn varchar (20);
ALTER TABLE mytable ADD newcolumn VARCHAR(25) AFTER secondcolumn;
ALTER TABLE mytable ADD newcolumn VARCHAR(25) FIRST;
Alternately, the column can be added with php my admin. Just add the custom field(s) and attributes and click Go.