Hello, and welcome to hecarim: The fast pony. Build movement speed, kill everyone. Gotta go fast.

Pros:
Good sustain, after w is maxed
Cons:
easy to die in jungle when not building tanky

Build movement speed.

Movement speed items ranked by movement speed bonus/cost:
Boots of swiftness
Alacrity
Youmuu's Ghostblade (When active)
Zeal (builds into statikk shiv, trinity force, and phantom dancer)
Zephyr (amazing item, rush if you want)
Twin Shadows (only buy for the movement speed bonus, use the active whenever)
Statikk Shiv (Love the passive, this item is literally the best)
Trinity Force (Too expensive, maybe buy if you are laning)
Phantom Dancer (The passive is silly, won't do anything, because hecarim's passive already does this)
Lich Bane (Too expensive, not enough movement speed. Don't buy)

You'll do fine

His w is the greatest jungling skill in the game, with the exception of mundo's burning agony.

Obviously take ghost, use to initiate, for the bonus ad
Smite when in the jungle, otherwise, possibly ignite, or flash.

Gotta go fast. What more needs to be said?

Just do whatever, honestly, this is easy for hecarim. Just go ad.

Jungle away

Use the ult when initiating, use e when on cooldown.

always work with your team. This is obvious.

Farm for your life.

Use w to heal, use q whenever off cooldown, and enemies around. Build movement speed. Win.