In Cyrodiil, if you joined the main campaign, most of the people are where there are assaults. Open map and you'll see where the battles are going on.

It goes something like this for groups or solo:

• Delve (solo, or group with friends, it's not an instance and there might be other people in there).
• Group Delve (found in Craglorn, same as above but actually requires a group).
• Public Dungeon (harder than the delves, can be done solo, but a group is recommended unless you're high level and geared well)
• Normal Dungeon (You queue up for these by pressing P and clicking dungeon finder if you're on PC)
• Veteran Dungeon (Same as above, but it's recommended that you're at least CP 160)
• Trial (People will invite for these in open world or in one of your 5 guilds, have to be max level)
• Veteran Trial (Have to be geared very well, have high CP and at least know what you're doing in the normal variations).