You want to look for picks at level 2, try to hide in a bush where they don't suspect you. Get an E off early game and it could be the lane winning kill, every time I've gotten a kill level 2 on the enemy ADC we've won lane because they would be too scared to push up anymore at the sight of a Morde with E. If you get ganked try to lure the enemies under turret and then hit W at max or close to max and you can probably get a kill. To play safe if losing lane, just try to tank projectiles with W when you can.

Late game you want to act as a tank, during objective fights like baron or drake target the biggest threat or the jungler depending on the situation, if your team can handle the biggest threat then ult the jungler (if it's not the jungler). Try to ult closer to when your jungler will have to smite because if they get out of your ult alive they can still smite the dragon or baron.