Start with 2 Mana Potions and a Harvester's Key. If you'd like, you can start with a Health Potion, but I personally find 2 Mana Potions help with Iggy's early game sustain a lot better which is crucial for him. Iggy is very fragile and burns through mana in the blink of an eye in early game. Play safe and try not to get caught right in the middle of team fights as you are not threatening to most heroes at the start. If you do get into team fights, support your team from afar with your turrets. Getting caught in the middle of the lane before you hit level 5 by the enemy team is almost guaranteed death since Iggy's base Health Points are meager. Once you unlock your second turret you immediately become the best minion clearer and will stay that way for the rest of the game. The combination of 2 Turrets and an Oil Spill allow you to clear Red and Blue buff camps very quickly, so don't be afraid to grab those if you have the chance.
As far as Card Points (CP) goes, you can dump your potions and key right at 6 CP to fully invest in a Magus Ward or Wellspring Staff, or tough it out until 9CP to keep a Mana Potion for a little easier sustain. It's fully up to you.
Priority of skill point investment goes like this:
- Turrets (5 Upgrades)
- Oil Spill (4 - 5 Upgrades)
- Flame Belch (2 Upgrades)
- Oil Slick (1-2 Upgrades)
The damage scaling on Molotvs is truly awful without Oil Spill, so you really don't need to invest anything in them. The second you hit level 5, unlock your Flame Belch. This is where Iggy starts hitting his stride.
Once you have 2 turrets and your Flame Belch your damage becomes quite vicious despite no investment. During mid game Iggy can burst down enemies very quickly if they are caught off guard. Your turrets will also shred through just about everything not named Sevarog and give ADC's a very hard time if they haven't invested in attack speed early on. This will last for quite a while, so start investing in your Tempered Plate and Tuned Barrier equally. Once your defense cards are maxed, you become extremely hard to kill and very annoying to the enemy team with your pushing capability. Remember, you are hard to kill, not impossible to kill. If you run out of mana you are a walking target and can't do much with your Molotovs. Seek Blue Buff whenever you can, melt through enemy minion waves and let them do the work to towers while you repeat the process to another lane.
As late game slowly approaches, your next investment is entirely up to you. I personally prefer in this order Meltdown --> Magus Ward / Wellspring Staff --> Staff of Adamant. Meltdown will allow you to seriously hit hard late mid-game. Wellspring Staff / Magus Ward (whichever you didn't choose at the start) extends your sustain. Staff of Adamant sacrifices some of your offensive potency to keep you hard to kill as you enter late game.
(Alternatively, you don't necessarily have to invest in Staff of Adamant in the same way I did. If you'd like, you can go full Major Casts to extend your damage or full Greater Health for extra tankiness.)
Iggy starts to fall off on the threatening scale once ADCs hit harder and Tanks get bulkier. Meltdown and Staff of Adamant allow you to stay relevant when you get into skirmishes. Luckily, Iggy doesn't fall off in the same way other casters do in late game. You are still the master of minion clearing + pushing lanes up ridiculously quickly, and with your bulkiness you can hold your own against other heroes without fearing getting immediately melted by ADCs. In late game you shouldn't be picking deathmatch fights like you might have in mid game. Instead, keep lanes pressured and other heroes occupied trying to repair the pushes you have started. Use your turrets and wards to your advantage to pressure lanes hard without getting ganked by roaming ADCs. While you don't have to fear getting bursted down instantly with your investment in defense, you will still be chipped away at very quickly. Play to Iggy's natural advantages and you will still be a huge annoyance to the enemy team even during late game.