Changing the default DarkRP job means that when a player initially connects to your server, they won't be placed in the Citizen job by default, they will be placed in the job you specified. This is useful for servers that provide an alternative type of roleplay such as StarwarsRP and MilitaryRP.
In addition to this, when a player is demoted they will be reverted back to this job.
garrysmod/addons/darkrpmodification/lua/darkrp_customthingsand proceed to edit the jobs.lua file.
GAMEMODE.DefaultTeam = TEAM_CITIZEN.
GAMEMODE.DefaultTeamvariable will be defined as
TEAM_CITIZEN, in order to change the default job, you must replace
TEAM_CITIZENwith the desired default job's team, for example