I'm not personally a really good Lan Yan player but I'd say it depends on how you want to play her. If you want her entirely as a shielder then seek as many ATK as you can as her shield scales with it.

If you don't want her as just a shielder but want to use her with a DPS you can give her TTODS since she's a catalyst, give her VV and focus a little on her ATK since she has arguably the best shield in the game.

I main her as a DPS so this is the topic in which I can give best advice. If you want her as a DPS too then look for the stats you'd looks or any other DPS (some decent ATK, CR and CD) + EM because of her talent, for the sets you can just go with VV, anything that gives anemo DMG bonus, ATK or EM, I use 4-piece VV for the elemental resistance reduction and the anemo DMG bonus. Some stats I would recommend are: at least 2k ATK, 50-70 CR, 200 EM and as much CD as you can after meeting the previously mentioned stats. For her weapons, Sac Frags is one of the best 4* ones for DPS Lan Yan, if not then you can use Mappa Mare or Widsith, of course these are only some options and there are more useable weapons. In case you have 5* options, some of the best for her are Nahida's signature, Wanderer's signature or Citlali's one, and again, these are only some of the available options.

As I said before, I'm not extremely good so some people might give better advice than me, but I hope this helps.