Most of the junglers usually start frog now, then go to their blue buff and then either recall or carry on to wolves etc. So after having done their Frog, and carrying on to blue buff they usually have 50 - 75% of their health and no smite neither health pots. So after they have finished blue buff they are left with about 200 health.
So, in order to get first blood or just steal blue buff early game, all you have to do is ward enemy blue buff at 1:40 - 1:50 and go mid lane to farm 1- 3 minions, then you should already go to their blue buff (obviously only if the jungler actually started frog, then went to blue) and just W over the blue buff wall - when the blue buff is below 100 health - (preferably hitting enemy jungler AND blue buff), if the jungler's health is low enough, just ignite him and do a few auto attacks, and youre done, you have blue buff and got first blood. You can freely Spam your q and w on the enemy mid laner.
But, if the enemy jungler is not low health enough to be killed by ignite and a bunch of auto attacks, just W back and go happily back to your lane with a blue buff.

Before you hit lvl 6 try to play more passive as to keep enough health and mana to gank bot lane. You can usually get a kill or two when your insane base damage of your spells (Easiest combo to do on enemy adc is q r w)
If the adc flashes away though (usually after you've already hit your q, just flash with him/her and R W following them, W back and you ll get a kill)
If you recall make sure you have enough gold to get essential items (1200 or slightly above/ 2060) or obviously 2800 for a complete Rod of Ages.
And try to keep an eye on the enemy Blue buff (usually spawns at around 7:30 depending if enemy jungler started Frog/Blue or Golems/Red)

Just make sure you roam A LOT especially if your enemy mid lane isnt easy enough to take a bunch of kills off.

Just make sure to always aim for APC/ADC's in teamfights, and stay out of cc/silence so that you can get safely back with your w.
In teamfights you have to act like ADC's, just stay at the back and do burst Dmg, if you prefer to act a bit more agressive (like myself), due to the Rod of Ages stacking quite a lot of health, you can.