Frostbane is a strong Juggernaut who needs a few levels to get going. He is a tank best paired with weapons that will keep him alive and disrupt the enemy. He can slow enemies with his basic breath and his secondary attack launches a powerful nuke to devastate enemies from afar and up close.

You are a tank so take something to compliment that like Aegis or take something to balance out your high mana consumption like Rising Soul. I play him as a Tank/Support focusing on durability before damage of any kind.
Frostbane at the beginning of a match is pretty squishy and it takes a 3 points in your health to make you as solid as a stone, so focus on killing minions and defending allies before you start raking in kills.
Use your abilities carefully Frostbane - is one of those girls you see in movies going on a shopping spree if you spam them, they may have low cooldown but your mana doesn't.
Do not let an enemy escape if you can catch him, since your Frost Breath slows the target, this makes it easy to catch up to them, once you hit them that is. If you cant catch your target just stop chasing, you are better to your team alive.
Ice Shard is a depth charge- At level 3 Ice Shard deals a lot of damage to a lot of targets, lead in with it if your enemies a clustered together. Just don't spam it too much, you may need that mana later.

Frostbane is good with Rising Soul, Aegis, Chillblade, Rainmaker and Stormcaller.

For Runes take Max health and Damage reduction and maybe some Life steal if you want, this should make you difficult to kill early and exceptionally hard to kill at late game.

For spells I recommend ironhide and heal for durability, Snare for disruption, Recall for leveling or Totem for buffing allies.