I will just add that it depends on where you're actually standing, as to the damage you inflict on a resonator. There is a 35 meter radius around a portal, 70 meter diameter...almost 210 feet. If you are standing on one side of the portal, and the opposition planted their resonators on the other side, you are wasting your ammo. A good indication is when you use your XM blaster, and only inflict 1% damage. Try moving around to another side of the portal and attack again. If it is still only inflicting 1%, there's a possibilty that it is getting remotely recharged. I find if after the third attempt you are STILL only inflicting 1% damage......time to move on. Go farm/hack some portals.
Talk to your team, find out who's been there the longest. Chances are they have literally hundreds of low level items that they don't really use anymore, ie lvl 1 resonators, blasters. Meet up with them, they can drop items in front of you, and you can pick them up. Team up together, let them destroy the portals, while you fill them up with resonators, followed by linking.....if you want to level up fast, that's a good way to do it.
Just some tips.