Army created - check. Army upgraded - check.
Army composition and upgrade status are important, likely the most important decisions to make, however don't overlook the other options available to you namely Character Items (Armor, Weapons etc.), Spells, Runes and to a lesser degree Facebook only enhancements; these can all turn a good army into an unstoppable force.
Items falling under this banner are Armor, Belts, Footwear, Helmets, Shields, Weapons and accessories (rings and 'treasures'). Items can be bought in the Royal Store (for a lot of gems i.e. real $) found in Lucky Chests and/or Pirate Treasure Chests (only available when the 'Treasure Hunts' are initiated) or (the most standard way) crafted.
Items come in various forms and are level bound for crafting (no such limit on bought or 'lucky' finds i.e. you don't actually need to be the level listed to wear the item, only to craft it), so generally speaking combat against players of a similar level will mean operating under the same limitations.
Items can offer significant advantages in Attack/Defence, Resistances and Leadership and are well worth the investment, though don't expect to craft Champion or Dominator items overnight, these items require resources (many of them rare) and even other items in order to craft.
When starting out the first two questions to ask are - what items can help my army specifically (consider Faction, Class and Tier) and what am I prepared to compromise on?
Hopefully the first question is easy to answer, with some players even basing their army on the items they expect to craft. Don't overlook the bonuses available to resistances, these can really add up but are often limited to 'Artful' items which are tough to craft (20% chance) so don't count these in your reckoning - in fact don't count any Artful items into your plan and consider them a 'bonus' when you get one.
The second question of compromise is related to the first, however there will be a strong likelyhood of going to battle with mixed factions and classes so an item that helps one unit may not be much use to another - choose wisely.
So you have a plan together and have your 'dream set of items' planned, how do you go about achieving your goal? Patience - and lots of it. You may need to get an intrim item or two before you get the item you planned for, either because you aren't high enough level or you simply can't get enough resources to craft the one you want. Don't be afraid to get a lower level item for a while, its better to have something in each item area than nothing!
Next look at what resources you need for your grand plan - concentrate on one item at a time, look at the Item Workshop and Items Dropped by Units sections of the Wiki to determine what you need and which creatures are most likely to drop that item. All the resources can be gained in single player bar the Chevrons, which are only offered as rewards for PvP combat, so if you want those you'll need to win some battles or at least completely destroy a single unit (only Legendary units can drop Officer Chevrons).
Final bit of advice - only start considering Champion or Dominator items when you hit lvl 30, not only are these items not craftable till then, there are a number of items that will hold you in good stead up till that stage and these items are often necessory to craft these elite items anyway!