Vs. ad get cloth armor 5 pot.
Vs. ap get boots and 3 pot.

Enter top or mid lane and spear the other guy. Dont spam spear at level one just shoot one or two on him. When you get to level 2 shoot a few more spears. Dont use w at all yet! Just spear unless you get a gank then you can w. If he gets really low after your spears just w-e-q-ignite. You should be good with this plan, its pretty simple, if you need help just ask for a gank. If you need to go back, you should first get dorans and start building brutalizer but make sure you can afford boots and a ward.

If your team needs saving ult into the other team and go for a squishy. If they focus you your team should be able to finish them off or at least do something :D You are very strong mid game so try to catch people overextended and stun them with w so your team can get a kill. Dont go first into fights. Wait for the tank to initiate and then do something.

Now it is your team to be really useless! Hopefully your early game was good... Dont run into fights... just wait till everybody has blown their ult then w into their biggest threat. You will get focused but you have to do something. It's gonna be hard to set up an ult late game but if you find a good chance then do it.