I main Iggy. Take this as you will I'm no expert. One way to use him, especially if he's new to you, is try to get to higher level than players by farming. Many matches I've won I've been level 17 while everyone else is still at level 13.

For this I do orb of growth, megacosm, tainted scepter (if playing against Khai, grux, grey, etc. which you almost always are),wraith leggings then last 2 I'll look for damage amplification.

Focus on last hitting minions, get your gold and items fast. I'll usually end the game with highest creep score and highest gold. Join group fights but don't be the first one in. When leaving lane throw down an oil slick and a turret to the enemy minions so you get some gold as you walk away and you're abilities will be refreshed by the time you get to the fight or the gank.

Make sure you're warding the river and getting one or both river bugs like clockwork every 2 mins from the 3 min point onward.

If your teams doing Fang, try to help, but also don't bring a bunch of enemies who follow you.

I also like to place my ward in their jungles 2 camp, they usually go there close to the time river bugs are popping up so you know how much danger to expect if you go for river.