Your mana pool is way to low. I started healing heroics with 22k mana with kings and i didn't had any problens healing it. I would focus on int gear first, with 14k mana you will go OOM really fast, because divine plea won't be enough.
And about "if you can tank you can heal". Jesus, stop saying that, to be a good healer you need to be a good healer, not a tank or dps. If you are a tank and a good healer also, grats, but you will find thousands of great tanks that can't heal too. You can be a healer or not, just like tanks or dps, you can suck on something and be good at something else (personally I do suck at dpsing, always agroing -.-)
For heroics i would use all mana cooldowns and glyphs, divine plea, seal of wisdom, sacred shield on the tank full time and you should be fine.
Raiss ~ Holy Paladin @ Gurubashi Horde.