Welcome to the most OP new meta. Smite Fizz middle lane is potentially one of the best duelists in the game, with the damage amplification from his ultimate and challenging smite.

Start with your E and head on up to the raptors camp, proceed to kill the little raptors first, they should die in about 2 shots (after landing E), smite the large one and use your potion. Your E should come back up about halfway through fighting the large raptor, which you can use without resetting it if you are fast enough. After killing the raptors you should head on into lane, missing about 3 minions maximum, and have a level advantage on your opponent.

Core Items

Finish your jungling item as soon as possible, so that you can smite your opponent and get the AP and CDR. Next you want to pick up your sheen and some boots for movement speed and some burst, finishing both as soon as possible. Alternatively if you come up against a heavy AD midlaner such as Zed or Yasuo , you may want to build Zhoyas first or in certain situations were you need some early MR, you could consider a Abyssal Sceptor, though I would almost always recommend finishing your Lich Bane first. Following that, you will want your Death Cap for some extra Damage.

Situational Items

If your enemies are building alot of MR, get a Void Staff to increase your damage against them, Will of the Ancients is a very unusual item, only build it if sustain is of the utmost importance. Nashors Tooth is a personal favorite item of mine, as it maxes out your CDR, makes the most of your W with increased attack speed and has an on hit effect that can be applied by your Q, all around i think it is a great item on Fizz.
As for defensive items, build a Guardian Angel if you are diving in a lot and the enemies have a good spread of damage, Frozen Heart if the enemy team is heavily AD based and Spirit Visage for heavy AP. Both the Frozen Heart and Spirit Visage provide CDR which you will need with this build.
Finally buy a Ludens Echo if your team needs greater Wave clear and you feel like a bit more damage.

Mostly explained in the chapters above, but in summary, take smite for damage increase and early jungle camp, flash for escapes or plays, alternatively you could take ignite for greater kill pressure.