Really it kind of depends on where you started.
Qeynos area: Qeynos lawn trash is good until about 5, but if you get bored you can go into the aqueducts as early as 2. It's like a maze down there but some good kills and newbie loot. Good experience modifier also.
http://wiki.project1999.com/Qeynos_Aqueducts gives you the complicated map. A great super newbie dungeon that isn't utilized much.
Blackburrow is good from about 5 on, and the gnolls in and around Blackburrow drop gnoll fangs that can be turned in at the Warrior's guild in Qeynos for very good XP. (The moonstones you get from that fang quest can be turned in for faction and a much smaller amount of XP.)
You can also kill lawn trash in the Qeynos Hills until 10ish.
10+ you can stay in Blackburrow or move into West Karanas and kill bandits for sashes (more XP and faction). Plenty of other things wandering around there too.
Freeport area: West/East freeport lawn trash until 3-4. From there you can go to kill more lawn trash in either North Ro or East Commonlands, maybe group up for orc camps in East Commonlands. You can also do Befallen from 5 on, very tough dungeon off of West Commonlands but best experience modifier in the game, and doable if you can find a friend or two.
Befallen will keep you until your mid to late teens if you want, but there are also dervishes to kill in West Commons as you gain levels, and orcs and crocs in Oasis of Marr in the early to late teens.
Kelethin area: Orcs on the lawn outside Kelethin until 3-5, then Crushbone until you can't stand it any more, basically. Unrest nearby is probably good from 12 on, or even earlier if you are brave or have friends. If you just want a change of scenery there are some goblins in Butcherblock, and the mobs on the chessboard.
By the time you get done with those things you should be in a position to go exploring on your own, or you can solicit some more advice about where to go and what to do.