What to Upgrade First

For TH6 people getting ready to upgrade their TH to 7, make sure you have AT LEAST one max upgraded barracks so you can start upgrading to dragons as soon as the TH finishes. That with the laboratory upgrading should be the first two things you start on followed shortly by spell factory. Start upgrading dragons to level 2 as soon as lab and the first barracks are finished.

In addition to the things Richard put up, these are a few you should give fairly high priority to:
spell factory (not dark; when applicable): that extra lightning spell makes a huge difference for dragons, GoWiPe is impossible without 3 spells
army camps (stagger upgrades whenever you can afford the elixir)
clan castle (when applicable; at least until lvl 3)
barb king (when applicable; at least until lvl 5): makes farming easier and is an extra tank unit for war