Novice to First Job
As a novice, you may start by fighting porings, lunatics, drops, and pupas at prt_fild08 which is the lower warp from Prontera.
If you are choosing archer as your first job, start your journey at Payon and hunt for willows as early as level 1. This will make your job change to archer easier.
If you are choosing merchant as your first character, pick up loots to sell to the NPC as part of your quest demands for a payment of 1,000 zeny.
After your job change, you may choose to grind your levels or use the eden board quests. Either of them is good. If there are people playing with you, it is better to form a party to level to take advantage of the +10% exp bonus per party member.
A common leveling area is hunting wolves 2 maps down from Payon. With your mercenary, you can kill wolves with ease. Beware that attacking a wolf will aggro wolves in the vicinity so plan your attacks well.
You may also go to the ever trusty Payon cave one map above Payon.
After gaining a couple of levels (usually when you reach level 51 and you aren't able to use a mercenary anymore), you may move to the following maps:
Archers, Mages -> ein_fild04 (1 warp above Einbroch)
Thief -> xmas_dun01/xmas_dun02 (Toy Factory)
Swordsmen/Merchants -> orcdun01 (Orc dungeon)
Acolytes -> form a party with someone :)
After turning to your 2nd job change, you may continue to level in those sites til you reach level 75 and be able to join most leveling players in their party in magma dungeon.
If you want to go solo, you will need some decent equips to go to these common level sites:
Hunter -> Sting @ Glastheim Culvert (gl_sew03)
Assassin -> Anolian (cmd_fild03)
Sage -> Seal (cmd_fild08)
Knight -> Minorous (moc_pryd05)
Crusader -> Raydric, Raydric Archer (gl_knt01)
Priests -> join a crusader or knight in their leveling :)
Monk -> Sleeper (ein_fild08)
Wizard -> Party up in magma!