If you own the island, go to World-->Region/Estate-->Environment and choose the settings you want people to see.
If you own only the parcel, set a landing point and route teleports to it (About Land-->Options), then rez a transparent, phantom prim at the landing point with a greeter script inside—something like this:
default
{
state_entry()
{
llVolumeDetect(TRUE);
}
collision_start(integer num_detected)
{
integer i = 0;
while ( i < num_detected)
{
if (llDetectedType(i) & AGENT)
llInstantMessage(llDetectedKey(i), "Welcome, " + llDetectedName(i) + "! Please set your sky to sunset.");
++i;
}
}
}