This is how i start Swain. the Doran Rings give great early gank hp, ap, and regen. After Dorans I recommend using a build site because I customize my build for the enemy team (and dont wanna spend all day on this lol). that goes for the runes too, but I recommend the penetration runes for early ganking.

I start tryin to gank at level 3-4. I open with a Never Move on the lowest hp enemy, then if it hits I combo Torment + Decrepify + Ignite and stay in range to auto attack.

Note: try to bait the enemies flash, cleanse, and ghost before using ignite in the full combo.

This combo does a ton of dammage early and even later games can take half the hp bar of a lower level enemy or squishy when combined with Swains ultimate.

Good Luck!
Belltolls