How does this work?

Okay, so far I bought GW, and GW Factions, and close to buying Nightfall.

I added my keys to my account. Can I make a character from factions or nightfall and start in Tryia?

And my other question.

If I get a char from Tyria to Elona or Cantha, can I access the features GWF and GWNF have to offeR?

First question: if you want to make an assassin or ritualist, you have to start in cantha. if you want to make a dervish or paragon, you have to start in elona. in tyria, you can only make 1 of the 6 core proffesions.

Second question: yes, but you have to start the campaign from the beginning of it, just like a canthan-born or elonian-born character would have to, even if you are level 20.

You start from where a level 20 should be. For Factions and Nightfall, a newly reached level 20 is expected to have completed only the starter island.

Okay I made a character in Factions, how can I get that person to Tyria so i can do the GW:P missions?

To take your Factions character to Prophecies:

Play through the Factions story until you reach the mainland, and take the Chaos in Kryta quest from Guardsman Chienpo in Kaineng Center. This extremely easy quest will allow you to travel from Cantha to Tyria (and you can cross back at any time by using the ship icon on your continent map).

You need to get to Kaineng so you can do a quest that takes you to Lion's Arch.

A little more specifically - since people sometimes get confused on what it takes to get to Kaineng Center and wind up paying for runs they don't need:

1: follow the primary quests and missions until you have finished the two missions on Shing Jea Island. Minister Cho's estate and Zen Daijun

2: After you have finished the 2nd mission you will be able to take a quest from the man at the docks in Saitung harbor that will get you to Kaineng City. You will arrive at Kaineng docks and the only exit from there will be the Market place.

3: From the marketplace go north to Kaineng Center (stay as close to the coastline as you can and you will only meet one group of afflicted - dont fight them - run through them or there is an alleyway on your right as you come to them that will lead you around them.

4: Once in Kaineng Center take the quest from Chienpo and follow the quest's instructions to get to Lion's Arch in Prophecies.

For more information, visit this link

http://gw.gamewikis.org/wiki/Chaos_in_Kryta

Do I have to be a specific level?

For crossing between Prophecies, Factions and Nightfall you can be any level. The only requirement for crossing is completing the quest that lets you make the crossing (and you only need to complete the quest once per character).

To add a bit more to this: As has been said, you can only start an Assassin or Ritualist in Factions and a Dervish or Paragon in Nightfall.
You can start one of the 6 core professions in any campaign, but, there are different choices for face, hair, colour, etc. in the different campaigns, and different starting armors. So you may want to check out the available "looks" in each of the campaigns before starting a core character.