What do you mean by "joining a city"? How does that work and what are the cities for?
For the online mode, you will talk to a cat on the southwest side of the village. Choose network mode to go into the online multiplayer. Solo port is just a room that no one else can enter and allows you to take both the AI companions you eventually get with you. It has all the same quests as the network mode and multi ports. Multi port is a local multiplayer mode. A wii U player can make a room that up to 3 3ds players can join. Unfortunately, two Wii u's cannot play locallysince they can't join local rooms, only online. So
Once connected, it will ask you to select a world. Each world has a capacity for 1000 people. Most people either go to Free A, or if they're looking to hunt G rank monsters, G rank A. You'll be starting at low rank, so basically go anywhere but the G rank and Expert Worlds. Not that anyone really uses the expert world.
After selecting a world, it will ask you to select a lobby. Each lobby can hold 100 people. you can go wherever you like so long as the lobby isn't full.
After that, you'll be given quite a few options:
Create a Room--Make a room and call it whatever you want for whatever you want to do. You can even password lock it
Enter a Room--enter one of the rooms already made in that lobby. Make sure you can do whatever their target is before entering. You can press R to see the name and hunter rank of the room's creator
Search for a room--Enter search criteria to find a room with the same target you want. You should usually check if there's a non full room doing what you want before making a room yourself
Move to a World--change which world you're in
Move to a Lobby--change which lobby you're in
Friend List--see if your friends are playing MH. If they're are already in a room, you can warp directly to them and send them messages from this menu
Short message--Shows the messages you've received from people during your current playsession
Chatlog--lets you scroll through the chatlog and see what people around you are saying
Exit-- End network mode
At anytime online you can press ZL to open the chat menu to type things into the chatlog. You also have some preset things called "Shoutouts" you can edit these to your liking so you don't have to type a favorite phrase every time.
Once you are in a room, you can also activate voice chat and speak with people. Keep in mind that the sound output from other people will come out of the gamepad by default (this can be changed in your ingame menu) so you need to have the gamepad's sound on to hear them.
The most important thing you need to remember is that whoever creates a room is the room's "Host". As long as they remain in the room it will remain open. But if they leave for any reason (turn off their console, log off, get disconnected) everyone else will get booted to their individual solo ports offline. Most people will give everyone a heads up before closing their room, but some don't. Try to be courteous about this. If you know you will be leaving after you get back from a quest, and there's a guy waiting in the room for you to get back to hunt, let him know so he doesn't waste his time. And also don't lie about how long until you finish the quest.
I once went into a room where there was already two hunters in a quest. I asked them how long before they got back. They told me they were almost done, so 5 minutes tops. I waited for 20 minutes for them to get back, and once they got back, the host closed the room. I could've gotten two quests done in that time if I had gone somewhere else. Some people are just plain inconsiderate.
Anyway, I could probably drone on for awhile but that's what you need to know before going online. Whenever you have questions I'll be happy to answer.