How to play Lancer as a tank in three easy steps:
1 - Use Challenging Shout once all mobs are gathered and continue to use it when it's available, but pay attention to surroundings and make sure no mobs have broke from you towards healer/dps and are out of range (close gap via leash or charging lunge)
2 - Utilize Stand Fast (block) and Shield Counter (gained at lvl 26) often to continue your aggro build. Shield Counter reduces damage taken in half, as well as negates knockdowns, so it's a far better skill to spam than any other damaging ability (and it has one of your lowest cooldowns for Lancers)
3 - Shield Bash when it's available and beneficial. If you're looking to speed clear trash mobs, you can follow up with Onslaught to dish out some decent DPS yourself (while still maintaining half damage and knockdown invulnerability), then go back to spamming CS and SF/SC. Against Bosses/BAMs, utilize Shield Bash to replenish mana, stack debilitate, or refocus the Boss/BAM on you after a nasty crit from your DPS or heals.
Additional Pro Tips:
Be smart with your leash--Don't use it as an opener unless it's providing a necessary advantage. Leash is a great way to close the gap between you and a mob that strays from the pack due to a big crit from your DPS or healer. Another thing to remember is that the leash ends in a one second stun, so while large mobs may not be affected by the leash's movement condition, they are affected by the stun, so once you've got their actions down you can utilize Leash to cancel out certain abilities they may try to use during a fight. This can be substantially beneficial to your group in boss and BAM encounters.
Charging Lunge can clip through NPC targets, so if you get knocked down or somehow end up in a bind where you're surrounded by enemies, simply point your nose to the thinnest population of baddies and Charging Lunge will be your savior.
Debilitate is amazing when going up against BAMs or Bosses, but keep in mind that a nine percent endurance redux means a direct boost to DPS, so don't slack on your shouts, bashes, and counters; if you see the boss headed for a group member, you can use leash to stop him in his tracks, which I've found typically gives you enough time to get his attention back on your shield and poker.
There's more to it as you go, but this should get you into instances without looking like a moron. If not, don't stay Lancer.