I highly recommend if you're new, to work through it slowly and learn your class!

Lvl 6-8: Goblin camp, deliver letter to Rosinden Quest, Relic Hunting
Lvl 8+: Rosinden Quest, Cryptkeeper's Quest, Gentleman's Quest, Piaf's Farm Quest (Requires Fodder or done later)
Lvl 15-20: Ogre Caves (No Demons)
Lvl 10-25: Graez
Lvl 18-25: Laerad, Parnelli Forest, Shadowlord's Forest, Fortress and Graveyard (Deku)
Lvl 23-33: Marsh of Fear, Azha Theatre Quest
Lvl 20-25: Demon Part of Ogre Caves, Ogre Cave Quest
Lvl 22-25: Kill Minotaur (Graez), Medusa (Graez) and Deathknight (Graez), Turn in Axe Quest Graez, Lothwaite Spirit Quest
Lvl 25-30: Dallyh forest, Obsidian Staircase, Kinaro Caves (good saves or fodder/AOE)
Lvl 30-35: Laerad Dojo Quest, Hound Camp
Lvl 35-40: Crystal Tower, Dark Marsh
Lvl 35+: Serakii farms
Lvl 35+: Scarecrow, Arkhon, Muln, Dream Dragon, Geonslu
Lvl 38+: (With Good Gear) Lava Tubes, Serakii Farms
Lvl 40+: (Without Gear) RP, Serakii Farms (Avoid large groups)
Lvl 47+: Tecqumin quest
Lvl 50: Dallyh Marsh (melee class or caster class with good spell knowledge)Mostly to help clear tax at this point/make money

Lvl 6+: 8 Goblin camp, deliver letter to Rosinden Quest, Relic Hunting
Lvl 8+: Rosinden Quest, Cryptkeeper's Quest, Gentleman's Quest, Piaf's Farm Quest (Requires Fodder or done later)
Lvl 10-22: Graez, Shadowlord's Forest, Fortress and Graveyard (Deku), Azha Theatre Quest
Lvl 18-25: Laerad, Parnelli Forest
Lvl 20-30: Marsh of Fear
Lvl 20-25: Demon Part of Ogre Caves, Ogre Cave Quest
Lvl 22-25: Kill Minotaur (Graez), Medusa (Graez) and Deathknight (Graez), Turn in Axe Quest Graez, Lothwaite Spirit Quest
Lvl 25-30: Hound Camp, Dallyh forest, Obsidian Staircase, Kinaro Caves (good saves or fodder/AOE)
Lvl 30-40: Crystal Tower, Dark Marsh
Lvl 30-35: Laerad Dojo Quest
Lvl 33+: Serakii farms
Lvl 35+: Scarecrow, Arkhon, Muln, Dream Dragon, Geonslu
Lvl 38-45: (With Good Gear) Lava Tubes, Serakii Farms
Lvl 40-50: RP, Dallyh Marsh (melee class or caster class with good spell knowledge)
Lvl 48+: (Without Gear) RP, Serakii Farms
Lvl 47+: Tecqumin quest

*GRAEZ: Only take on regular zombies and skeletons, avoid other enemies, till at least 10 lvls more than listed.
*Death ward feat or spell, is a must, recommend getting it around 10-20.
*Lvl 35-50: Dallyh Marsh (If you have a level 50 Friend) (ALSO FODDER IS VERY HELPFUL HERE)
*Leveling in a party adds some bonus exp gain, up to 4 members.
*Relics turned in by anyone in a party gives exp to all members of the party.
*Lvl 6+ Do demiplanes daily, clear out all enemies and boss if can.
*Lvl 20+ Start doing daily Wailing Isle quests, they are short and easy, and exp scales with level.
*To get more in game help, type rumors, this command will list level appropriate areas and quests as well.

There is a lot more of the game to explore and the rumors command will show it. These are just the some of the more straight forward and simple places to get to and/or complete.

Fighter, Paladin, Ranger, Barbarian, Lunar Oracle, Nature Oracle and Cleric can do most if not all things 5-10 levels earlier.
Fighter, Paladin, Barbarian usually require around Level 21 with PRC, then they can steamroll.
Life Oracle can do most things 8 Levels earlier.

help rewards
reward <character name>

This gives usually around 10% a level to the person you reward, exp tax takes a cut of course.

Always nice to reward people for their time and RP!

Collecting relics requires game knowledge, and locations across the game. Though lots of time you can run into them or find them during quests.

At a 'Collector of Lost Relics' read list (This will display locations of relics for your level)
Go collect relic
return to collector
claim <relic>

1-10 Level Difference: No Penalty to slight.
11-15 Level Difference: Greater Penalty
16-20 Level Difference: Severe Penalty (Not Worth It)
21+ Level Difference: No Exp

*Quest Completion EXP does NOT get affected by this.

Thanks Eilistraee and Swus for help!

*Some classes and or combination of groups can do things sooner or later! This is just a general guide to help.