Honestly until you get 50 and start storing plaques for the t1 tanking gear you're really going to be hurting for tanking. Start saving up honor too, because two of the best tanking items (shield and mace) you can get until raids come from honor and you have to buy the lower level ones first to get the higher level ones. Concentrate on spellpower, wisdom, endurance, block (dodge/parry) and work on your spec a bit. Cleric tanking is pretty gimp right off the bat at 50, because our armor is so much lower than warriors and we have less damage reduction based off of cooldowns and innate resists. You'll probably need your support to actually help heal for a while unless you get awesome healers.
A lot of people take Dauntless Courage in the Shaman tree instead of Unyielding (which you did). I disagree with this choice, because a critical strike increase coupled with Overwhelming(shaman2) and Walk In The Light(sentinel2) make for a huge damage output and self healing increase. Increasing your chance to crit also increases your total self and party heals, significantly increasing your mitigation. Glacial Shield is good for before a pull, but rarely useful in rotations because it generally equates to less healing than an AOE melee swing, even with the recent increase to damage shield scaling with spellpower. This means until 50, you should probably just put a point in Overhwelming instead. The increase to crit damage and heals is much more useful as it increases your threat generation, damage output and passive mitigation through self and party healing.
Supremacy(Justicar6) is useful only for PVP in most cases. Most bosses can't be stunned, and when you stun something it generally only serves to make your gathering and LOS of casters/ranged more difficult. Many people take interdict, but its range is so poor and casters tend to immediately start casting again, so there's no point. Take the two points from Supremacy and put them into Vengeful Justice(Justicar4) so you can tank more things at once in a pinch, get more damage output and thus more passive mitigation through healing.
You don't need any points in Commitment(Justicar6) since you have two mana cooldowns between Ageless Ice(you get it next level) and Purpose. You can put those points into Hammer of Virtue(again increasing threat, damage output, and self healing.) I don't usually go much further into Justicar than this with my tanking experiments, since the armor difference seems to not be worth the wasted points in useless abilities like Rebuke(you can just LOS stuff and tell your party to wait a second on pulls..)
I don't usually put more than 38 points into Justicar in any tanking build because that is all you need to get Just Defense, an excellent cooldown to use during pulls on a short timer.
Example 42 Build: rift.zam.com/en/stc.html?t=00nrj.Veb0zM.yuosVecboo
Example 50 build: rift.zam.com/en/stc.html?t=00nrj.Veb0zM.yuosVecboo.V0V
Also you can see my tanking spec on this forum: rift.zam.com/en/stc.html?t=10nrd.ced.yuosVecboo.Vtob
This last build (my build) comes with a great tanking armor buff, a mana drain, a great damage AOE for threat in Soul Drain, Purge for utility, Perserverance for a nice fear canceling cooldown, Sanction Heretic which is great for your single target macro, and Vex which is great for gathering since it heals you with Salvation(Justicar) and Contempt(Inquisitor3)