The energy decay speed for different resonator levels

Resonator distance from the portal center is determined by your distance when you place the resonator. Stand 50m away from the portal and the resonators will be 50m. Stand 1m and the resonators will be 1m away.

When using the "deploy" button without selecting a resonator slot, it will select one of the open direction slots and deploy there. I believe it begins with the one closest to you, but I'm not 100% certain

Hack plenty of enemy portals. You get 100 AP for hacking an enemy portal, but none for hacking a friendly one.

If there are no unclaimed portals around (highly likely in some locations), find a level 1 enemy portal and start slugging away. You'll want a few dozen XMP bursters at minimum to destroy the portal, but the rewards are high--destroying 8 resonators and planting 8 of your own on a single portal will get you almost a quarter of the way there. Even if you don't manage to capture the whole portal, you'll still have gotten 75 AP for each resonator you destroy.

Deploying portal shields is a good way to get AP that may not be obvious at first. Check every friendly portal you encounter to see if it has a full complement of shields. If it doesn't, deploy as many as you can. You get 150 AP for each one.