Instances require either a Shared Task (Alt+Q), Monster Mission (Alt+Q), Adventure (Alt+V), or Expedition (Alt+Z). All of these types require a minimum of 3 people (including mercenaries) to obtain.
The original instance zones were based on LDoN (Lost Dungeons of Norrath) Adventures. Expeditions start with GoD (Gates of Discord) and are generally for 60+ level groups, so probably not for you yet, but later on, you might look into the MPG group trials. There aren't too many group Expeditions, most are designed for raids.
Shared Tasks there a many, but hard to give any good lower level examples for me at the moment. Higher level ones, such as the DoDh (Depths of Darkhollow) tasks and spell arc missions are some of the best though. The DoN (Dragons of Norrath) progression missions are also great ones.
Monster Missions are a combination of shrouding (playing a monster or other type of NPC) and a Shared Task. There are lots of these, and can be done at any level. You can find a list in the link to the left under Quests, called Monster Missions. Rewards scale to your toons unshrouded level when you are finished.
As far as experience rates go, it varies alot by mission, expedition, adventure. LDoNs are descent xp at lower levels, and wins (along with appropriate NPC hails, and the like) will evolve your Adventurer's Stone charm aug. Vendor loot is descent from them as well, as weel as some LDoN-only tradeskull supplies, although they're not too popular any more for bazaar selling. All the other types, vary alot by the type, and particular mission. The High Hold Keep Monster Mission used to be awesome xp, but it was nerfed many years ago.
Most people do certain Missions, and Expeditions, for the rewards rather than for experience, as it's generally better xp to stick with good xp zones, and hot zones. LDoNs even aren't awesome xp, but are generally easy at lower levels, and give descent xp.
Again, I don't remember any lower level shared tasks that are great, but there are severl Monster Missions, (since they're do-able at any level), that give out some great rewards. The Dain series in particular, but there are many others as well. The DoDh spell arc missions, and DoN progression are also great rewards, and particular ones of the them are good xp, specifically, the 69.1 spell mission.
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Forgot, for higher levels, the Mechanical Guardian instances are great for xp, as well as chance to get some good loot drops (and quest drops for various quests including the face aug series).
You might also want to look at this overvew of quests
Yther Ore. Edited, Mar 24th 2009 11:47pm by Yther