This link is a good list of zone(s) appropriate for your level. Questing in those zones will always be faster than grinding (all of the leveling guides for WOW involve strategically chaining together quests, and for good reason: it's the fastest way to level.)
If you're pulling unwanted adds against even-level opponents, you should use your ranged weapon. Eventually you'll get a feel for which mobs need to be range-pulled and which can be Charged (range pulling everything is inadvisable, as you miss out on all that free Charge rage).
For a typical class I usually try to face (and do quests) level +0 to -2 mobs. But for warriors I felt like -1 to -3 was a little better (so at 40 that'd be level 37-39 mobs). It just felt like critting was more beneficial for warrior DPS than other classes, due to white crits meaning more rage to turn into more DPS. You crit more frequently against lower level mobs.