Plenty of activity left sure
As for the realms, you're right there are 3.
Which to choose is really down to your playstyle etc
Hibernia is usually the underpopulated (although not right now, they're dominating everything)
However Hib is usually quite an elitest realm to get grouped on and will probably cause you alot more frustration than it's worth
Albion and Midgard are usually the realms better suited for "zerging" (following 1 guy with 100+ players and mindlessly running over anything in your path)
I don't play Mid/Alb too much myself but I've found they are much more friendly towards unknown/new players etc so to be honest with you, I would go with one of those.
As for class... well there's alot to choose from and each unique
Plenty of people will jump in and correct me as to what I suggest being the wrong choice for a new player but I would maybe start life on something like a Heretic on Albion
It's a class that you can easily level up by yourself if you have to, has good DPS, can buff itself, doesn't die in 2 shots, you have heals etc. Just a nice all round class to be honest.
Everything is viable however some are less viable to run around by yourself on
i.e a wizard - It has really nice DPS but anything that attacks you in melee will drop you pretty quick. Not such a problem if you're in a group but if you're leveling by yourself it will probably be quite frustrating for a new player.
There is a 5 minute timer to swap between each realm but given how easy it is to level up these days you might just find yourself grinding a few of the characters up to the 20's to see what they're about.
Takes about 2/3 hours to get into the 20's just by yourself, even quicker if you can get a group.
-----signature-----

Coreline 9l1 Ranger & Corlina 11l1 NS

Currently ~ Corella ~ 3l3 Bard

Ranger vid ~
http://tinyurl.com/Coreline
NS Vids ~
http://tinyurl.com/CorlinaVids