Yes, the PvP helm is actually the best you can get pre-raid so don't fret it, high stamina, high armor, high mastery, it's basically everything you could hope for in a starter helm.
I think if you're just getting started and you're fine in normals but struggling in heroics, there's a pretty simple explanation. In normals you're likely only to meet brand new 84s/85s who are just gearing up as they work towards heroics. In heroics, you're liable to meet ilvl 360+ folks looking for a quick fix of VPs. The difference is like night and day, you can hardly even compare the two.
Masterpoobaa is right, a little concession to Hit/Expertise might help you get off the ground while you're gearing up. Making sure to tell groups you're new and just learning definitely makes a difference, too. You'll get some arseholes who will make noise when you say it but that just lets you kick them in advance or leave and spare yourself the trouble, nice groups do actually exist on LFD and most of them are happy to see ANY tank.
Keybind Skull and Cross markers to F1 and F2, or something convenient that you can reach quickly and easily. Use them liberally! Mark Skull even in one or two mob pulls, that'll get the DPS in your group used to the fact it's going to be there on the bigger pulls. Be consistent about using them, too, don't just throw them onto things at random. Keybind some other raid markers near them, too, for CC and make a macro you can click at the start that explains what you want, something along the lines of "/party CC: {circle} = Polymorph {diamond} = Hex {star} = Sap", etc. or just type it out based on what you have available in the group. Learn what CC all the different classes can do, and what target types they can/can't use it on!
In DM the trickiest trash pulls are the ones with Defias Blood Wizards + several others. The Ragezones (red bubbles) buff the mobs' DPS meaning you have to pull them out of them (until you're madly overgearing the place) and if your DPS are conscious enough to run into it suddenly they're doing insane DPS.
I'd recommend getting an addon like Vuhdo, configuring the threat indicator in it to highlight the frame for a party member that has aggro and bind Righteous Defense to middle click, then place it somewhere you can see it easily. Get TidyPlates and the Threat Plates plugin, when a bar turns yellow change to targetting that mob. Stun it if you have to, Divine Plea a Shield of the Righteous into its face, get it back under control, switch back to primary target. If you have problems with people consistently hitting the wrong targets remind them about the marking scheme.
For the ring: Ring of the Battle Anthem! Buy it with JP, it's MUCH better. Problem solved.

Let us know how it works out.
