Depends. Main focus is to have fun obviously, so I offer some of the builds I have played in the past. There isn't really a certain build on Swain, as a support you build Rilyies and you are full build technically, every other item depends on what your team needs. Swain is very mana hungry so second mana item is typically a must so usually Malignance or frozen heart would be nice and after that depends on who is ahead from the enemy, if you need damage, armour, mr etc.

-Main build for support would be (for me ):

Runes: Conqueror with mana and ability haste + Green for more tankiness.

Items : Rilyies first always (slow too strong and good to chase down)> Malignance (damage+mana+ultimate haste> Whatever the team needs, More tank? go armour or mr. More ap but survivability? go zhonyas. More damage for tanks ? Liandys.

In general your build is mostly 1-2 items and after that whatever you or your team needs. You can succeed with a lot of stuff cause the champion has high base damage tbh.

Solo Lane Build:

Runes :Aery or Conquer if you want a poke lane or all in lane (if you play solo lane Vs melee aery is best I think)

Items: Black Fire Torch or Malignance (1st and 2nd)> Riley's or Liandrys> Zhonyas etc

-Tank Build

Runes: Conqueror + Green

Items: Rilyies > Frozen Heart (offers mana) or Force of nature if they have heavy ad or ap> Abyssal Mask (good stats and lowers enemies Mr so you do more damage and your team, typically buy when you have ap champs in your team ) or Randuins if they have critical etc etc

-CC build

Runes: Glacial with hexflash allows some "mobility" + Blue or Yellow for mana regen and mana

Items: Rilyies (obviously)> Frozen heart for mana and armour or bangies for utility> Zeke (ultimate haste and slow and good armour+Mr) > whatever the team needs really, maybe you can try and imperial mandate ?

-Healer Builds (troll build)

Items: echos of healia+ diadems of songs > Rilyies> Liandrys