What do I do?

So, i've leveled a few characters to like 18ish, and ended up deleting them just do to not really understanding mechanics after not playing for like 6 months. I haven't reached 20 before so I don't really know what to expect when I get 20 sometime soon on my necro. I was wondering what sort of things people do once they have gone through and done all the questing and missions for a campaign, ive heard of like prestige armor and heroic modes and i was wondering what those were and what the benefits of getting them are? I was also wondering what is a good campaign to transition to eotn from, and what level should be attained for eotn.

Prestige armor has no benefits other than they are better looking and more expensive.
Hard mode is unlocked as soon as you finished that campaign and then becomes accessible to all characters once those reach level 20.
To transition to eotn I my opinion there isn't really a campaign that is better to transition to eotn better than the other since they all start you off at the same point in Eotn.
You need at least level 10 to access eotn and once in eotn your attributes you will be automatically buffed to that of a level 20 including armor rating and such.

There is tons of content after lvl 20 and you will reach that well before you finish the basic game missions.

Really you should play the game if you enjoy it and stop when you don't any longer.
Its a game not work.

So finish the game missions then any other quests that prove interesting.
If still hooked and you have the other games cross over to those and finish them.

Change your secondary and see how that works
Try to gain all the skills for your class then if you like all the others.
Explore the elite areas.
Go for titles Like mapping the whole game world.
Try to capture and tame all the animal companions and fill your menagrie.
Farm for fun and profit.
Play the game in hard mode and vanquish areas ie kill all the enemies.

That should keep you going for a while.

edit
You can transition to eye from any campaign and all take you to the same place so its really no matter which you choose.

Most of even a single campaign is done after you reach level 20. And you can take your character between al 3 chapters and EOTN (if you have purchased them), that is many many hours of play. After that there are the dungeons of EOTN and all the elite areas. FOW, UW, The Deep, Urgoz, DOA. Also, after you complete EOTN or prophecies you get access to the new War in Kryta stuff.
Not to mention pvp. You can jump right into pvp with any lvl20 char, but in high end pvp you are required to have specific classes and skills. As a new player you should start by trying alliance battle, jade quarry or Fort aspenwood, they are not so demanding.

So, it is actually like 95% of the game that takes place after you are lvl20.

Yeah, you clearly have a long way to go since you typically will hit level 20 fairly early in most of the campaigns. Eye of the North had a lot of content for you too.

My biggest regret was staying too focused on my first character and never even TRYING some of the others. Nowadays I get a lot of enjoyment rotating amongst ALL of the GW professions on a regular basis.

If you reach lvl 20 - well, the real game begins. All that happened before was some kind of tutorial. In Prophecies it is a bit different, since you reach lvl 20 somewhere in Maguuma Jungle. That is halfway through the campaign. In Prophecies and Nightfall you get to lvl 20 shortly after you reach the corresponding mainland, after the next 1-2 missions.

However, the enemies keep getting stronger while you advance through the story lines. Your task is now getting stronger by aquiring better skills and developing better builds. There are elite skills available as well you should capture. Try to get as many skills (normal and elite) as you can and improve your builds. Get armor with maximum armor for your class.

There is a final mission for each campaign - I assume this is the most obvious goal to reach.

My goals for each new character are to never skip anything in the storyline, do every quest at least in the tutorial section to reach level 20 early, play every mission in each campaign including bonus and finally learn every normal and elite skill for my primary profession and each available pve skill as well.

This concludes this characters development. From there, this character is ready to do everything else, for example elite missions or hard mode areas.

Glad I wasn't alone in this feeling. I played my main toon for far too long before enjoying other classes. Now I tend to play those before I jump on my main.

If you are getting bored before hitting lvl 20, you might just not like the game. There is a lot to do beyond hitting that lvl but you need to enjoy doing it.

Once u finish a campaign, u could play it again in Hard Mode - do all the missions in HM.
You can farm, hunt titles and just chillax....

About the best campaign to reach eotn pretty fast, i would suggest Factions since you lvl up pretty fast there and you only need to be lvl 10 to get to eotn.

Thanks a ton for the all the help. Its good to have an idea of what I have to look forward to once I continue through the game. So far I'm having a ton of fun, and it looks like the game gets even better! One last question, was wondering where I would get the best armor from as a necromancer? I've noticed that you craft gear in towns rather than from drops from mobs or bosses.

That is correct, you craft your armor. You should however make sure that you craft yourself some max armor (otherwise you might end up wasting your Ka-Ching updating your armor all the time).

The max armor for a Necromancer is 60.
Max armor can be crafted at the earliest in:
Droknar's Forge for Prophecies
Kaineng Center for Factions
and Consulate Docks for Nightfall.
Or you can craft it in Boreal station, the very first outpost of EotN.

Good luck, hope you'll enjoy your time to come with this great game.

Don't waste gold buying armour till you reach an armourer that crafts the max protection al 60.

Use collectors to get free armour till then.

You can view all the available styles of armour on guildwiki and choose one that you like, you can even take parts from different styles to mix and match your own style.