tell me what server pack you use l2j ?
if you use l2j go gameserver --> config --> l2jmods.propeties
and find this :
# ---------------------------------------------------------------------------
# Welcome message
# ---------------------------------------------------------------------------
# Show screen welcome message on character login
# Default: False
ScreenWelcomeMessageEnable = False
# Screen welcome message text to show on character login if enabled
# ('#' for a new line, but message can have max 2 lines)
ScreenWelcomeMessageText = Welcome to L2J server!
# Show screen welcome message for x seconds when character log in to game if enabled
ScreenWelcomeMessageTime = 10
and change for your welcome ;)