League of Legends (LoL) Question: can someone help me with the legendary item for sona

I personally recommend you buying Staff of Flowing Water or Ardent Censer as your first legendary item in every game (Depending on If your team is more spell heavy or auto attack heavy). Then you can go for more of a support -ish playstyle with items like Redemption and Vigilant Wardstone - you can also take Mikael's Blessing if enemies have a some serious CC abilities in their team like Dark Binding or Sleepy Trouble Bubble. You can however also go for more damage/snowball oriented build with Mejai's Soulstealer and Lich Bane - here you can take Zhonya's Hourglass or Banshee's Veil If you need more self defense (depending on what you want to defend yourself from).

For more information, you can check some of local sona guides or u.gg for example.

Have a beautiful day. I hope I was able to help <3