Even if you are not an experienced coder, looking at the source code always helps to answer questions like that. For instance, looking in the do_train function (in act_move.c) shows us this:
/*
* Check for trainer.
*/
for ( mob = ch->in_room->people; mob; mob = mob->next_in_room )
{
if ( IS_NPC(mob) && IS_SET(mob->act, ACT_TRAIN) )
break;
}
if ( mob == NULL )
{
send_to_char( "You can't do that here.\n\r", ch );
return;
}
So, this puts us in the right spot. do_train is called when you type "train", and there is the error message "You can't do that here.".
Now we need to see what condition is met, to *not* get the error message. The ACT_TRAIN bit is needed (that sound logical).
So in the Area Editor you would set the "Train" flag in the "Actions" tab for the mob that you want to make a trainer.
For your other question, the room for new characters (I presume you mean new players), if you look in comm.c you see that when a new player is created this line is done:
char_to_room( ch, get_room_index( ROOM_VNUM_SCHOOL ) );
Also in merc.h we find this:
/*
* Well known room virtual numbers.
* Defined in #ROOMS.
*/
#define ROOM_VNUM_LIMBO 2
#define ROOM_VNUM_CHAT 1200
#define ROOM_VNUM_TEMPLE 3001
#define ROOM_VNUM_ALTAR 3054
#define ROOM_VNUM_SCHOOL 3700
#define ROOM_VNUM_BALANCE 4500
#define ROOM_VNUM_CIRCLE 4400
#define ROOM_VNUM_DEMISE 4201
#define ROOM_VNUM_HONOR 4300
So it seems clear that new players will be put into room 3700. To change that, change the line there to make 3700 into something else, and recompile everything, or at least the files that use ROOM_VNUM_SCHOOL (comm.c and db.c).