Dungeons give a lot of XP, one run is worth many cards (kind of depends on the dungeon but this generally applies). Daily missions are not bad either for a long time. You can start doing those at level 100 in Fedimian.
It does not really matter when you pop your cards but sometimes you can gain a little bit of extra xp by not using them immediately. I think you start losing out XP when the monsters you kill are +/- 20 levels away from you. So try to stay closer to the map level you’re currently doing. It’s not really a must-have though. Like I said one dungeon run gives shitloads of xp, worth way more than the little bit you gain from waiting with the card usage.
Popular dungeons are at level 50, 90, 115, 130, 145, 175, 190, 217, 240, 290. The only exemption is the level 160 dungeon. Don’t even bother queuing up for that one. Literally nobody does it (for whatever reasons, probably crappy xp and drops). The 290 dungeon is also a bit special in that people don’t queue up for it but rather group up via shout chat. Mostly because the queue isn’t very intelligent and sucks at grouping people up due to level differences. This is especially true for the 290 dungeon.
At some points in the story you will find that there are several maps to quest on (for example after the mage tower at around 120). So you may have to look around a bit. I suggest using this map here to find your way around:
http://toscamp.com/map/map4.1.php?lang=en
Can’t really think of anything else of the top of my head right now. If you ever get stuck with your level though just do daily missions for a few days. Best time is right after the reset or in the evening when people return home from work.