*****STARTING ITEMS*****
i. Starting items 1: Use it when you are up against and easy champ and weak early game
ii. Starting items 2: Use it when you up against an AD champ such as Zed, Yasuo etc.
iii. Starting items 3: Use it when your up against a strong early game champ such as LB or against a ranged champ who can harass with his/her basic attacks such as Lux, Brand, TF, etc...
***** MID-GAME ITEMS*****
i. Aether Wisp will help you roam and gank other lanes easily this will be helpful early game.
ii. Luden's Echo must be your first item except against zed. Luden's give you Massive damage and MS that will help you get more kills and won't let your enemies escape even if they flashed (sometimes).
*****CONSIDERABLE ITEMS*****

Buying these items will help you deal additional damage and void staff will help you ignore 35% of the enemies' MR. So if the enemy team have tanks with heals or Health regen. such as illaoi, mundo, cho etc.. consider buying these items to help you in killing them with your team.

PROS
Easy to Master
High Mobility
Nuker
Good Harassment
Easy Ways to Escape
Easy farming

CONS
Very Squishy
Hard to catch up if you were behind
Becomes Vulnerable When E is on CD
Can be easily countered
If your ult is stopped, you're dead

COMBOS USED IN LANE

You can Harass by Q all the time until he's out of mana or health pots and then engage by QEW.
Whenever he's on low HP and low mana, engage with the same combo + R (QEWR) and ignite to secure the kill.

TIPS THAT WILL HELP YOU IN EARLY-MID GAME

Whenever your enemy is on low Hp and he's under the tower, you can use this technique to take him down: (Flash--> Q --> E --> W --> get the kill and E again to escape and do not get killed by the turrent).

Whenever you are pushing try roaming, especially bot lane, you can go bot and take down the adc because he's squishy and if he is killed, mosly the supp is useless like (Janna & Soraka etc..)
This way you might get a double or help your adc get the kills.

You might not be able to take down the enemies top laner alone, so whenever you see him pushing hard, gank top and ask for your top laner to engage and use your combo (QEWR)

If your enemy team is having a squishy jungler in early game (Evelyn, Shaco, Graves etc...) ward their jungle and try killing them when they're having low HP, this way you'll get easy kills and avoid getting ganks because their jungler will probably be farming.