cmd:startjob(playerid, params[])
{
    if(isplayerthis || isplayerthat) //Examples
    {
        new number = random(6); //We have 6 jobs to choose from, so that's what we use. (It is possible to get 0)
        switch(number) //Remember, you must have yournumber + 1 in the random slot. Example: We want 5 options, so we do random(6). We want 3 options, we enter random(4). etc.
        {
            case 0:
            {
                //Truck Driver
            }
            case 1:
            {
                //Taxi Driver
            }
            case 2:
            {
                //Medic
            }
            //etc. case 3, 4, 5...
        }
    }
    return 1;
}

case 0:
{
    If (playerdata(playerid) jobid == 1); //trucker in this case
    { 
      Etc etc
    }
} 
case 1:
{ 
 ETC
}

CMD:startjob(playerid)
{
    new job = random(10); // This will get something between 0 and 9
    switch(job) // Make a switch for job, it works like else if
    {
        case 0:
        {
            // Things that will happen if the job is 0...
        }
        case 1:
        {
            // Things of job 2...
        }
        // And there you'll go...
    }
    return 1;
}