1.) All 2nd job quest npc's are located at the 2nd town. You need to do a pre-quest at level 15, if you remember when you reached lv15 a message should have popped up about it on your screen (it's also done on 2nd town). Once you're done with the pre-quest, you can immediately start your 2nd job quest.
Each character needs to talk to a different npc to start their 2nd job quest. For Elsword it's at the accessory shop npc, if I'm not mistaken Raven is at blacksmith and eve at alchemist (I forgot the rest).
Here's a screeny of Elsword 2nd job quest (credits to mausmani for pic):
*It's the blue titled quest. Top is Sword Knight (巨劍騎士), bottom is Magic Knight (魔法騎士).
2.) If you count the base job as your first then yes, Magic Knight is Elsword's 2nd job and his 3rd job would be Rune Slayer. As for Raven it's Sword Taker then Blademaster. His other job would be Over Taker then Reckless Fist.