I always start with ruby crystal because his passive will also give Singed health as well as mana. If I'm laning against ranged enemies and they're holding me back, I'll usually just keep behind my minions and wait for them to kill all of the minions from my wave so I can have their minions follow me to my turret through my nasty fart poison. Once I have 950 gold, I recall and get my catalyst. However, if I'm doing relatively well I'll usually just stay until I need to b. After catalyst, I get my boots of swiftness as soon as possible and then strait to Rod of the Ages after that. I find on average I'll have my Rod by around 18-20 minutes; I've noticed that if I can by it by 8-12 minutes, it means I'm several kills deep I just end up pooping on my enemies with my nasty poison and flip.
If my team and I are doing pretty well, I'll usually get Rylai's Crystal Scepter starting with Giant's Belt so I can slow people with my poison. This is really good for Singed because it pretty much makes you or and teammate you are with impossible to catch, as long as you keep the enemy in your poison.
On the off chance I happen to have more deaths then kills or assists, I'll usually just get defensive items according to what the enemy team mainly consists of after Rod of the Ages. Just remember that Frozen Heart is a really great armor item for Singed because all the mana with turn into health as well.
As far a abilities go, get your poison first (always), then flip, the mega adhesive. After that just max out your poison and do what you will with the rest of his abilities (granted you always get his ultimate when you can).
Follow these steps and play it cool, and it should take their entire team dumping all their abilities on you for a good ten seconds to actually kill you. Have fun!