Olaf is a funny Champion, but not very beginner friendly. You should have already played an Attack Damage Champion. On the whole, Olaf is one of the AD'ler that can, because of his high health, plugging much.

Wrigglers Lantern:This is a good Item for early game, beacause you get Armor,Damage and Life Steal.

Mercury's Treads :Magic Resistance and Tenacity.

Frozen Mallet:This item gives you Life, Damage and a Slow.

Phantom Dancer:You always need Attack Speed and Krit's.

Madred's Bloodrazor:Armor, Damage and Attack Speed. Nice Item.

Guardian Angel:

Good Summoner Spells

Exhaust: Exhaust lowers the damage output of your target and greatly reduces their movement speed. If you are engaging an enemy, Exhaust is definitely something you should throw on them.

Flash: This summoner spell is good for any champion. Flash allows you to close the distance between fleeing enemy, dodge certain enemy abilities, correct positioning mistakes, or teleport over certain obstacles. If you don't take this, prepare to be punished hard when you make positioning mistakes.

Ghost: If you have problems with overextending, or need to cover a distance larger than Flash would. You could use this summoner spell, I recommend this for newer players until they develop the "gut instinct" that they are in danger and should back off.

Ignite: Ignite is amazing for finishing off that stragglerh as Ignite deals true damage and cuts healing in half for its duration. If you are in a 1 on 1 battle against another AD carry, you can immediately ignite them to reduce their lifesteal; however, in this situation Exhaust would be better as it reduces their damage output which in turn, reduces lifesteal.
I have recently noticed that many bottom laners are now taking Heal. Taking Ignite will counter this as Heal's effectiveness is cut in half if you cast it before they Heal.

Heal: Not the best choice, but it's ok. Heal can be used to turn a losing fight into a winning one when people go way too far into your territory to kill you.