When farming, always try to last hit minions to gain farm and gold. It helps if you stay behind your partner a bit to keep safe and not take damage. If you think you cannot last hit properly, just try to auto hit but still try to kill minions with the auto hits.

I would recommend taking Heal & Flash. But if your partner gets heal, I suggest you get Exhaust & Flash. (If you're paired up with a support, it's more better for you to take Exhaust rather than heal)

In team fights, don't just stay back because you're afraid to die. Go up a bit and try to focus the squishies or if you have madreds, go for anyone you can get your arrows on. Remember, always stay close to the support and behind the tank for maximum damage output, if you find your enemies are focusing you, try to run back until they stop, then go back in for some damage.

When you start, your items should be A Doran's blade for the little life steal and damage.
If you feel you are having trouble in your lane you should probably get a second doran's blade.
Mid game/Late game you should maximize your damage output. By getting your boots of course (Berserker's Greaves) Then getting a Infinity Edge, Bloodthirster, Black Cleaver, Madreds Bloodrazer and an Elixir of Fortitude and Agility.

Focus (Passive) While out of combat, Ashe's Critical Strike chance increases every 3 seconds for her next attack.

Frost Shot While active, each of Ashe's basic attacks slow her targets. This drains Mana with each attack.
Toggle: Ashe's basic attacks slow her targets by 15/20/25/30/35% for 2 seconds.
Costs 8 Mana per Attack
Range: 600

Volley Ashe fires 7 arrows in a cone for increased damage. Volley also applies Ashe's current level of Frost Shot.
Ashe fires 7 arrows in a cone, dealing 40/50/60/70/80 (+) physical damage. Volley also applies Frost Shot.
Costs 60 Mana
Range: 600

Hawkshot Each time Ashe kills a unit, she gains some extra gold. Ashe can activate to send her Hawk Spirit on a scouting mission.
Passive: Ashe gains 1/2/3/4/5 extra gold whenever she kills a unit.

Active: Ashe animates a hawk to scout for her, revealing terrain as it flies toward target location.
Cost: No Cost
Range: 6000

Enchanted Crystal Arrow - Ashe fires a missile of ice in a straight line. If the arrow collides with an enemy Champion, it deals damage and stuns the Champion for up to 3.5 seconds, based on how far the arrow has traveled. In addition, surrounding enemy units take damage and are slowed.
Fires a large arrow in a straight line. If it hits an enemy champion, it will deal 250/425/600 (+1) magic damage and stun that champion for up to 3.5 seconds, based on the distance the arrow traveled. Additionally, surrounding units take half the damage and are slowed by 50% for 3 seconds.
Costs 150 Mana