how to play him then? I want to be able to use him effectively and surprise the stuff out of the enemy team.
step one is you have to play like you don't give a forget. Towers are bitches and do no damage. Run move speed quints and ghost for bonus style points.
Step two and perhaps the most crucial is to learn how to ward top side jungle to proxy. If you're in a bad matchup, like Darius, Riven, or something else annoying, you'll want to go behind the enemy's second tower and farm minions between second and inhibitor. The best start for doing this is the dark seal and a refillable potion. You can go corrupting flask first if you want, though. If you see their jungler coming on your ward, you have two options: go for a spicy bait or run for the execute into the inhib tower.Only bait if you know you can kill, because if they get one single hit on you, no more execute.
Singed isn't a great duelist, so your strategy is to farm and chug your flask when you want to trade or get low. In matchups that aren't that bad, you can lane normally and against melees, bullying with poison is doable and effective. Don't really use e or w unless you're sure you can kill. When using q, turn it on and then off as soon as it's on the unit you want to poison. It saves a lot of mana and nearly negates singed's early mana problems. Remember though, your primary goal is to farm so you can be really annoying later.
Core items are probably ryalis and liandrys. After that you can build whatever you want. This combination is evil because you can melt tanks down or destroy squishier champs with high base damage. Singed struggles with getting kited but if you get on top of their adc with rylais, they have no choice but to flash or die.
Last step is laugh and dance. Always laugh and dance. Bind them to easy to reach keys. It's essential to winning as Singed. Laugh always. Dance sometimes. That's how to play Singed on a basic level.