There are a number of big 6-man instances you will encounter as you level (there are also a bunch of smaller ones from the Epics and group quests but I will just go thru the big instances), I will try to describe most of them in order. If you like instance grouping and this is your first toon I would recommend to try to get a fellowship together and get them done, they are very challenging and can be great fun. If you are really up for a challenge you should try do these without help from a higher level person but sometimes after so many hours and wipes you may need help
You will get very good XP by doing these and some good loot, the XP from completing large instance quests is usually more than regular quests and this holds true all the way to the Radiance Gear instances in Moria. Keep in mind that most of the best gear lvl 1-40 is crafted crit (most). There are some good armor sets at around lev40 you can get from Tal Brunien quests (Trollshaws) that compare to crafted crit but most of the quest gear is not as good, not untill u get to lvl55+ and can start doing Radiance Gear instances. You will need to have Radiance gear to fight The Watcher, Radiance counter acts Dread effects and almost all instance bosses give off a certain amount of dread, some more than others, so bring Hope Tokens. The Watcher is at present time what you could call ¡°End Game¡± (which should soon change because Turbine likes to give us free content (<3 Turbine).
GARTH AGARWEN:
Lvl32-35 6man resetable
Located in the LoneLands just east and north of Ost Garuth, full of Elites and a very nasty place with tough bosses. There is a quest hub there but there are also quests that come from an NPC in the south of Harloeg near the Epic Book 2 ruins. There is a quest chain that I think comes from the old forest for the Red Maid, its been so long since I have done these, I forget so you may need to look it up. Lots of Wights and Ghosts.