First of all, all races start building walls from their central building (for orcs, the grand hut). Orcs have two types of walls to build, a light palisade and a heavier wooden wall. The walls will build automatically; but orcs are the only race who can send their workers (laborers) to work on the walls to make them build faster.
As for getting overfilled on resources, all races have technologies that can increase the storage space of your town. Orcs can research these from a hut (a regular hut, not the grand hut); and the human unit you're talking about dosen't add storage space, I'm pretty sure it just serves a a drop off point (like huts do for orcs).
Orcs are kind of weird in the way they train units: workers (laborers and maraurders) spawn over time from the Grand Hut (you can switch what is spwaning by clicking on the button). Then, you can send the workers to certain buildings (warhall and pyre) to transform them into basic infantry. Then you can send the infantry to a temple to turn them into heavy infantry, or to a warg pen to turn them into cavarly.
Other units are goblins (who spawn from the goblin tent, and you can switch what is spawning), wargs (who spawn from the warg pen as baby wargs and grow over time, and can be eaten as well), and ogres (who can be trained, not spawned, from the ogre pit).
Siege equipment can be created by sending workers to the armory, where they turn into whatever unit is selected (some need research first).
And as for what units you should have...well, this is just my opinion, but I love the warg riding cavarly; they're fast, strong, and come with a heal ability. Ogres are good too, especcially when you level them up really high. Orc archers arn't the best, but maraurders can double as corpse looters.
Well, that's all I got. Hope you enjoy the game!