You can start a party with somebody already in your friends list by pressing 'n' to open the ignore tab. Switching to the first tab of that menu will show a list of friends. Right click the name of the friend you want to add and select "Invite to party" from the drop-down list.
If you want to add a stranger to your party, you left-click their avatar/character once. A little bubble with their portrait and information should pop up. Right click their portrait and select "Invite to party" from the drop-down list. You can also invite them to your guild if you have that privilege, or inspect what they are wearing, amongst other things.
Have fun, hope this was helpful. There's also a forum of newbie guides, so I also recommend you go look at that. Here, have some links.
1: The actual forum for newbie guides ( which is mostly empty, but may have what you want.) 2: A Guide-thread I just stumbled across that might be helpful. 
