Yeah, basically, head down to the underworld. Look for the demons carrying a voodoo doll of the guide NPC. I'll not spoil too much more but erm... avoid getting it hot shall we say?
(boss fight)
Once in hard mode hard mode mobs do instantly spawn everywhere. The surface is still... mostly ok (until you enter ice/desert/corrupt or... well it's the opposite of corrupt) biomes then you'll generally find harder enemies spawning
It's not like instant death by a long way, if you have good normal mode gear you'll be fine.
Once in hard mode - the... well... altars do.. stuff
It rotates in 3's (will all become clear) but each will spawn more patches of corruption etc in a random point in the world so don't go crazy
(I usually do 6 altars). Hard modes a lot of fun though, especially with all the event bosses/etc added.
I usually go ranger/melee so nights edge, full molten armour, minishark, phoenix blaster, necro armour and I'm ready for hard mode
(though I usually keep full meteorite armour + space gun handy for basic exploration as the space gun doesn't use ammo once in full meteorite).
There are some AMAZING extra's in hard mode though. Teleporter pads, homing bullets, rocket launchers as a bare/crap example
Wiring teleporters to various areas makes getting around a lot faster. Always one to dungeon, jungle, temple etc.