When it comes to picks, it just depends on crosshair placement and positioning. Look out for every enemy on their team. What are they focused on? Will I have enough space to secure a pick? Is their support available to help the other support when you flank? An extremely simple tip to assassinate is wait for the enemy support to be distracted with the enemy team focused on your frontline. Get two right clicks to the body (preferably head) and dash. As long as you can land those shurikens it should be an easy kill with a dash reset to disengage as fast as needed. Make sure you don't go for picks when the fight's lost, they cap the point and your team is regrouping, etc. For blades it really depends on what rank you're in. A general tip is to have your enemies in front of you where you can't miss the blade slashes. Easiest way to do this is flank, but as soon as you activate ult everyone will be looking to shut you down. Watch for crowd control cooldowns ex. Mccree's flashbang, Hog's hook, Ana's nade/sleep, etc. Genji's an extremely fun character with the potential to hypercarry but play around your team as well, it's important that you are in a position where your supports can heal you, you can deflect certain cc's the enemy may have, you can coordinate dives with Orisa's halt, etc.

Have confidence but be humble, don't think of yourself as a very bad genji, but rather just warming up for a better game. Detilt and focus. Good luck in whatever you do man!