JTek, you can go into pvp by sending an army out on the worldmap, then clicking on the pvp button near your army information. The button only shows when you are away from towns. You can enter the name of the player you want to target or leave it blank for a random player. The game will match you with someone with a similar strength army.
You and your friend can work together by doing coop quests (attack on AI towns similar to your own) and coop sieges against the larger NPC towns around the world map. You can also send each other resources to help each other out.
Coop quests are available later in the game as your progress in the quest storyline.
You can attack any of the large cities if you are at war with them. Move your army to the city's location, click the Enter City button (near your army information) then go into Diplomacy, declare war and then exit town. There is another button near your army information that will let you siege the town (only shows when near a town you can siege). Click that then click Co-Op. You can enter your friends name to siege with them or leave it blank to invite another random player.
To send resources, add your friend to your Friends list - in the Social window (button near resource bar) go to the Online tab, find their name and click the Add Friend button. Once they accept your friend request (or add you as a friend manually) you can send them resources. In the Friends tab of the Social window, select your friends name and click the Send Resources button. You'll then get a window where you can specify what to send.