[abhinavdabral]

Yes, BMUK has given a good idea... when your login is sucessful, just do SpawnPlayer(playerid);

EDIT: And for tutorial mode.. you can use
1) Change player's world.. So that others wont kill or disturb him/her during the tutorial.
2) Use TogglePlayerControllable(playerid,0); to make the player freeze.
3) use TogglePlayerControllable(playerid,1); to unfreeze the player after your tutorial is finished.