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

Lvl 6-7: Goblin camp
Lvl 8: Deliver letter to Rosinden Quest
Lvl 8-10: Rosinden Quest, Both Muileann Quests, Daily Demiplane*, Piaf's Farm Quest*
Lvl 10-20: Graez*
Lvl 15-20: Ogre Caves (No Demons)
Lvl 20: Pirates Cove and Torm*, Daily Demiplane*, Githyanki Fortress
Lvl 20-50: Wailing Island Daily Quests*
Lvl 20-25: Azha Theatre Quest, Demon Part of Ogre Caves, Ogre Cave Quest, Graez*\\
Lvl 22-24: Kill Minotaur (Graez), and Deathknight (Graez), Turn in Axe Quest (Graez), Lothwaite Spirit Quest
Lvl 25: Medusa (Graez), Laerad Caverns Quest (Asgard)
Lvl 25-30: Dallyh Forest, Crystal Tower (Seneca)*, Kinaro Caves*
Lvl 30: Daily Demiplane*
Lvl 30-35: Dojo Quest (Asgard), Hound Camp
Lvl 35: Scarecrow, Arkhon, Muln, Dream Dragon, Geonslu
Lvl 35-40: Crystal Tower (Seneca)*, Dark Marsh
Lvl 38+: Lava Tubes, Serakii Farms
Lvl 40: Daily Demiplane*
Lvl 41: Lighthouse Quest (Graez)
Lvl 47+: Tecqumin quest
Lvl 50: Dallyh Marsh*, Daily Demiplane*

* = See Extra Notes at bottom of the section.

*Piaf's Farm Quest: Only do this if you have Fodder, can be difficult without.

*Graez Level 10 to 20: Only take on regular undead. Do not take on Greater Undead (Requires Death Ward Feat).
*Graez Level 20 to 25: Should be able to take care of all of Greaz undead.

*Pirates Cove and Torm: You are here to gather items for Wailing Island Quest. (What you need, you will have to figure out in game.)

*Daily Demiplanes: The exp from these are not great at lower levels. So I recommend just blowing throw them to kill the boss. I also recommend doing them in the later half of the 10 level blocks. So at level 10 you will be the strongest. Same with Level 20, 30, 40 and 50.

*Kinaro Caves: This one requires decent saves otherwise it will be rough.

*Crystal Towers Level 25 to 30: Do not take on the bosses at this level, just take on all the enemies within the Obsidian Staircase.
*Crystal Towers Level 35 to 40: Now you can make your way through and face all the bosses except for Intruder at the very top.

*Dallyh Marsh: This is one of the hardest places in the game. Requires melee class or caster class with knowledge. Mostly to help clear tax at this point and make money.

*Klauth: This is not for leveling, but to complete your journey and to gain the best gear, then this is what you will need.

*God Champions: The next step after Klauth are the God Champions to get greater loot.

*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.
*Wailing Isle quests 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 5% 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.