There are a couple different ways to go about it. I think the most common would be to make an NPC named "Server" and put it in some inaccessible area. Then just have it shout stuff when needed:
void main()
{
object oServer = GetObjectByTag("SERVER_NPC");
string sMessage = "THE SERVER WILL RESET IN 5 MINUTES.";
AssignCommand(oServer, ActionSpeakString(sMessage, TALKVOLUME_SHOUT));
}
You could also just send a message to all current players by looping through them and using the SendMessage function:
void main()
{
object oPC = GetFirstPC();
while (GetIsObjectValid(oPC))
{
SendMessageToPC(oPC, "The server will reset in 5 minutes.");
oPC = GetNextPC();
}
}
Hope it helps,