Ready to fight some enemies? Zoom out and look for some red symbols. Those are enemies.
If there's a group of players online, playing together at the edge of a group of enemies, teleport to them. That's a great way to pick up the ropes quickly, and playing with others is fun!
Otherwise, you'll want to start small. Find a small-sized circle or rectangle, and head there.
Once you see it, and you have a clear path to it, rush forwards. If you have crowd-control abilities, (if you're using the starting tank, that's ability #1), activate them as soon as you're in range of the enemy; this will obliterate all those pesky minions and put a good sized dent in the main fortress itself. Circle the enemy until it's dead or until your shields (the circle around you) get thin; if your shields drop, back off and wait a bit to regenerate health.
Once your enemy is dead, you should see some diamond-shaped things flashing on the ground. That's loot; just drive over it to pick it up.
There are several kinds of loot you'll be getting: gridshards (indicated by a golden number rising above your tank), prisms (square objects with stars and bars inside them; they'll fly to the right), and modules. We'll discuss loot in the next section.
Essentially, all encounters in the game boil down to one of three simple tactics:
Circle: just go around the enemy, faster than its gun can spin. You won't take that much damage. This tactic allows you to stay in the battle for a long time, but is less useful when there are enemies (either neighboring long-ranged fortresses or minions) taking shots at you from different sides.
Hit and run: run in, do some damage, and run out. It's the standard tactic against high-damage enemies, and coupled with burst DPS abilities (shotgun, bombs, etc.) it's pretty effective.
Sit on it: If you have a tank with a cloak, are up against a really easy target, or are in a group large enough to split enemy fire down to manageable levels, sometimes simply sitting on an enemy will do the trick.
Sometimes, you'll die. When that happens, you'll get a message saying you disconnected then reconnected to the game (there's no separate death message yet), you'll lose all the gridshards on you, twelve hours will be taken off the time-to-expiry of your items in short-term storage, your prisms will drop a level, and you'll be rather embarrassed. (If you had no idea what I just said, don't worry - I'll explain everything in the next section.) Make sure to learn from your deaths!
As you get more tanks and encounter more enemies, your battle skill will improve. With enough practice you should have no trouble soloing any stationary enemy in the game.
Be sure to take a look at the enemy articles on the wiki!