Starters Guide to Items

Berserkers Greaves: Buy these if your damage is based on attack speed, and if they dont have a ton of stuns (if they do you might want Merc Treads).

Mercury's Treads: Buy these if you are more tankish, or if you are fragile and really need to avoid specific disables. If in doubt, get these, you cannot go wrong with them and they are considered by most as the best boots in the game.

Sorcerers Shoes: Buy these if you are AP and are mostly concerned with max damage.

Ionian Boots of Lucidity: Buy these if you are mostly concerned with CD's that you wish you could spam. Mostly bought on support to allow more heals or on manaless champs.

Ninja Tabi: Alternate tanky boots to merc treads, their main benefit is their low cost. Buy if you need to tank physical damage and need boots fast.

Boots of Swiftness: DON'T BUY THESE. Community opinion is that they are very cost and slot inefficient.

Doran's Ring: Buy this if you need mana and are AP. You can buy up to 3 if you want more regen. Also provides a nice buffer of health against burst damage and ganks.

Philosopher's Stone: Buy this if you need mana but are not an AP. It gives gold so buy it earlygame if you are going to buy it.

Chalice of Harmony: Buy this if you REALLY need mana and cant seem to get a blue buff from your jungler. I mean REALLLY need mana. If you do really need that mana though, it can be really good.

Rabadon's Deathcap: Buy this on pretty much every AP if you need pure damage.

Rod of Ages: Buy this if you need less damage but also need a little bit of tankyness, such as if you have short range.

Abyssal Scepter: Buy this if you need mostly damage and a bit of defense from magic and at least someone else on your team will benefit from the MR reducing aura.

Rylai's Crystal Scepter: Buy this if you need tankyness and if you have some single target spells. Best if they have several melee champs without gap closers.

Will of the Ancients: Buy this if you have another mage that will benefit from the aura, or if you have enough health to not die in one burst and need survivability, or if you are getting into poke wars.

Void Staff: Buy this if important targets are getting high MR (~90)

Athene's Grail: Buy this if you got a chalice early, or need lots of mana and some cooldown reduction.

Infinity Edge: Buy this if you need pure damage, usually on AD carrys.

Bloodthirster: Buy this if you need pure damage and some sustain.

Wit's End: Buy this if you need attack speed and a bit of tankyness. Extremely common on melee AD, ranged will usually buy Phantom Dancer so they can kite better.

Phantom Dancer: Buy if you need attack speed and your survival depends on kiting or chasing. Ranged AD almost always buy one. Needs to be comboed with a damage item.

Trinity Force: Buy this if you need tankyness and damage (you are probably a bruiser), and you have damage skills with short CD's to proc bonus damage.

Atma's Impaler: Buy this if you are tanky damage (probably melee) and have over 2k health. The more health you have the better.

Last Whisper: Buy this if important targets are approaching 200armor.

Wriggles Lantern: Buy this if you need sustain and a ward for your lane. If you dont really need armor or the ward just buy a Vampiric Sceptre for sustain.

Randuin's Omen: Buy this if you need health and Armor and they have more than one autoattacker you want to shut down (which pretty much every team does). Remember to use the active ability. Buy Heart of Gold early and get money then build this.

Warmogs: Buy this if you need pure health, buy it earlier so you have time to charge it up.

Frozen Heart: Buy this if you have some health already and need armor, and they have more than one champ who relys on autoattacks primarily. Also great if CDR is needed on important skills.

Force of Nature: Buy if you need MR. Do not buy if you dont have any health to back up the MR.

Guardian Angel: Buy this if you are carrying hard and need to avoid dying. Best if you have some sort of ability that you can use to get out of the thick of things after reviving.

Banshee's Veil: Buy this if they have a powerful AP who can ruin your day in one spell. Or just need more magic damage survivability.

Spirit Visage: Buy this if you need a small survivability boost in midgame and have some sort of self heal (big lifesteal/heal).

Frozen Mallet: Buy this if you need the slow to help your team catch someone or to slow down melee's with no gap closer trying to get to your teams squishies.

Shurelya's Reverie: Buy this, its really good on anyone who isnt pure damage. Remember to use the active. Buy Philo stone early for money then build into this.

Aegis of Legion:Buy this on anyone who isnt pure damage who will be taking a few hits, earlier you get it the better it is.

Zekes Herald: Buy this if you have several autoattackers.

Locket of the Iron Solari: Buy this as soon as early teamfighting at dragon starts, so you can use the HP5 in lane and the active. Also just good lategame.

Quicksilver Sash: Buy this if you are squishy and cannot afford to be disabled at all. Remember to use it.

Boots 3 Healthpots: Go this if you will benefit from staying at range (if you have some ranged harass), or to avoid skillshots. This is an all around good start with good sustain and power, if you arent sure what to start, start boots3.

Regrowth Pendant 1 Health Potion: Go this if you arent as worried about just getting jumped on and dying early, maybe there will just be a bit of harass from both champs in lane. You probably wont go this unless you are planning on building a very fast Philo Stone to get gold and mana you need in lane.

Cloth 5 Health Potions: Go this if you are going to be extremely heavily harassed in lane.

After starting items, Top's will commonly pick up one or more rarely two GP5 items (Philo Stone and Heart of Gold), then maybe pick up a sustain item such as Wriggles or Vamp Sceptre if they don't have built in sustain and need it. Stacking one to three Dorans Blades is also common on more aggressive tops. Maybe they will buy either a Brutalizer for a bit of midgame damage boost, or spirit visage form a midgame survivability boost.

Boots 3 Healthpots: just do it, especially if they have skillshots you need to dodge.

After Starting items, Mids will commonly stack one or two (sometimes three) Doran's rings for mana regen/buffer health/lane presence with AP, before starting their first big item (usually Rabadons Deathcap or Rod of Ages). If you are planning on getting a Rod Of Ages, you will probably skip most or all of the rings and instead rush to buy a Catalyst as fast as possible.

Dorans Blade: Buy this if you have a healing support

Boots 3 Healthpots: Buy this if you have a non healing support, you need those potions to stay in lane.

After Starting items, AD bots will commonly stack one to three Dorans blade to allow lane presence and easy last hitting. A Vampiric Sceptre is very commonly bought as well to allow for more sustain, and you can also buy a Wriggles instead of the Dorans if you really need sustain. After this, AD will typically try to farm to either Infinity Edge or Bloodthirster, then pick up a Phantom Dancer.

Faerie Charm 2 Sight Wards Vision Ward 1Health Pot: Ward your lane and the bush if neccesary, pay attention to their support, and use your vision ward to clear their ward and allow for a nice gank.

After Starting Items, supports will typically try to rush and buy a Philosophers Stone and a Heart of Gold as fast as possible (along with basic boots) to get some sort of income (as they wont be last hitting creeps). The most important thing is to maintain ward coverage over the lane and dragon at all times. After GP5's, support will move on to buying aura items such as Shurelyas Reverie/Aegis of Legion/Zekes etc.

Jungle starts are usually either Cloth Armor 5 Healthpots, Vampiric Sceptre, Regrowth Pendant, or Boots3HealthPots. These starts are heavily champion and rune specific, so consult a real jungle guide for you champion, which should explain why a certain start is better than others.