Hi, this is my first build. And I chose to make it about my favourite (new) character in the League : Orianna

For summoner Spells I take :
Teleport and Flash

Other spells that can work :
Clarity
Ghost

The others are no good for Orianna

For masteries I take the standard mage masteries (9/0/21). With 3 points in Archmage's Savvy (for AP, duh), 1 point in Deadliness (to be able to go to the next tier), 4 points in Sorcery and 1 point in Archaic Knowledge for the usefull magic penetration
In Utility : 4 points in Perserverence, improved teleport, 4 points in Awareness , 1 in Greed , ...

As first skill you should always take Command: Attack. Use it for scouting and damaging enemy champions. My second skill is Command: Dissonace. Your 3th skill depends on the sitation : is your partner taking to many risks or are your opponents aggressive, take Command: Protect. Else take Command: Attack again.

The skill priority is in a normal situation :
Command: Shockwave > Command: Attack > Command: Dissonance > Command: Protect

When you're losing in your lane :
Command: Shockwave > Command: Attack > Command: Protect > Command: Dissonance

Items :
Start
Doran's Ring

first recall
Build or buy Catalyst the Protector

Second recall
Boots : Ionian Boots of Lucidity


Third Recall
finish Rod of Ages : boost of health, mana an AP
s
Fourth
tart build towards Rylai's Crystal Scepter. It's give you 500 health, a nice boost of AP and adds a slow to your spells.

Fifth
Continue building or finish the Scepter

Next Recalls
Archangel's Staff : mana, mana regen and AP based on mana
Void Staff : Nice AP + magic penetration
Zhonya's Hourglass : If you're getting owned by AD champs
Rabadon's Deathcap : Epic AP

Last Items
I've never bought the last item. Never had the chance for it.
Your choice =)

Another option is to start with Sapphire Crystal and health pots. Then build it to a Archangel's Staff

Early game, just last hit the minions.
Early Mid game : use attack on the casters and after that use dissonance. If they're still alive finish them with Auto-Attacks.
Mid to late game W+Q should kill them.