Pick doran ring and pillar of flame. long range of your autoattacks and pillar spell, combined with about 40 AP in the beginnig and some nice mana regen with runes and doran ring gonna give some hard time for ur opponents.
With a decent lane partner you are in a good position for a first blood as soon as u've got all the 3 spells. Harass with just the pillar before 50%. After that, if u've managed to make the pillar-sear stun combination, use ignite and conflag and you are most likely gonna get a kill with ignite and ur passive.

your most reliable damage spell is conflagration, so I intend to max it first.
for clearing the waves use pillar + conflag on the burning creeps. after lvl 3 conflagration these 2 spells will get ranged ones killed.
When fighting champions, use pillar of flame, then try to stun the target with sear and finish it with the conflagration. If the target is alone, your ulti is not gonna cause a lot of damage, so try to use it when the target is close to a minion or just save it for a teamfight.

Get some cd reduction early on with the codex - as a combo-based caster u are willing to minimize the time spent waiting with nothing up to give the enemies to taste =)

rylai's is a nice choice for Brand - slowing enemies for your ulti gonna help maximizing the damage done by it in teamfights, plus, just as Zonya's and Abyssal scepter it will give u some durability and an awesome active to survive the focus.