For starting items you would typically get a Doran's Ring. You would want to get the Doran's Ring becuase it gives 4 mana every time you kill a unit. You also would want to get 2 Health Potion.

If you are taking this as a "support" (becuase you would be building full AP), you would want to grab a Spellthief's Edge instead of a Doran's Ring, and you grab an extra Health Potion, or just grab a Refillable Potion if you use Potions often enough, and you also back often enough, Refillable Potion would be a good item to get then. When I take this support, what I tend to do is get Eye of the Watchers for my Spellthief's Edge upgrade, the other option is getting Frost Queen's Claim, but I tend to get Sweeping Lens or Oracle Lens so I can clear wards.

Remember that if you are getting a lot of kills/assists and not a lot of deaths, then you want to get Mejai's Soulstealer for that AP that you can get from Mejai's Soulstealer.

The next item that I get is Rod of Ages because it gives up to 500 health, 400 mana, and 100 AP. The passive Eternity is another reason to get Rod of Ages because you will be mana hungry a lot of times in the game, so having that passive that gives mana equal to 15% of damage taken gives you a lot of sustain because your Aria of Perseverance gives you some decent health, which in term gives you some decent sustain in lane.

After getting Rod of Ages I typically get Lich Bane for extra damage with my Hymn of Valor auto attack combo I end up doing, but there will be more on that later. The Lich Bane is an essential part of this build. The explination will go a different chapter, so I will link that chapter for you to (link to chapter with explination of combo)(not done yet).

Once I get my Lich Bane, I usually will start getting Archangel's Staff. The reason you get Archangel's Staff is because it gives AP based on 3% of your current mana.

After getting Archangel's Staff, you want to get Rabadon's Deathcap for that 35% extra AP, plus that 120 extra AP from the base stats of Rabadon's Deathcap.

The next item you would be getting is Morellonomicon, main reason is for the extra mana, which because of Archangel's Staff, gives a bit more AP, its only like 12 AP, but a base of 100 AP. the Morellonomicon also gives 20% Cooldown Reduction, which will help you later in the game.

The final item you get will be Luden's Tempest, you get 100 AP from the Luden's Tempest, and you also get 10% Movement Speed, as well as the passive, which is not too good in my opinion, but it does give some decent AP, as well as the Movement Speed. If you got a Mejai's Soulstealer earlier, this is the item that you skip and just not get, because the passive is not too good on Luden's Tempest, and Mejai's Soulstealer will give more AP anyway, assuming you can get the kills and assists to sustain it, and not die a lot.

http://na.leagueoflegends.com/en/featured/preseason-update/8200-8229?build=21202156

you wanna get as much ap as possible, and as much CDR as possible while still getting a lot of ap, for long games gathering storm helps a bit, and you get ravenous hunter for that q damage