What is the best build for Elden Ring if I want to play as wizard.
Elden Ring has a wide verity of builds for a magic based character. However, there isn't any clear best build without a more specific kind of magic build you wish to make, as many potential types of magic builds available which may be enjoyable based on personal taste. However, I can recommend some magic based builds using the Version 1.06.

1.  Mage: This beginner build focuses on casting spells from afar with magic spells. Starting with the class of Astrologer and the starting item of a Golden Seed for an additional flask at the start of the game. Your primary stats are Intelligence for your spells and the Mind for additional FP to cast your spells. For your secondary stats Vigor is important to survive damage and Endurance to increase your equip load for heavier armor. Focus most of your flasks on FP with one or two HP flasks. Any staff should be fine for this beginner build along with any armor you are able to equip. For your spells equip Glintstone Pebble, Glintstone Arc, Great Glintstone Shard, Carian Slicer, and Carian Piercer.

2.  Spell-Blade: This build is focused mainly on melee combat and using spells to deal damage close up. Starting with the class of Prisoner and the starting item of a Golden Seed. Your primary stats are Intelligence and Vigor. Your secondary stats are Dexterity to allow you to wield dexterity-based weapons, and Mind. Suggested weapons are the Demi-Human Queen's Staff and a Estoc or Rapier. Your armor should ideally raise your Intelligence stat, but otherwise wear any armor you wish. An Ash of War Skill that can be put on your weapon is Impaling Thrust if you wish. For your spells equip Scholar's Armament, Glintstone Pebble and Carian Slicer.

3. Death-Knight: This build mainly utilizes the Sword of Night and Flame, along with death sorceries. This build can work with any class; however, I still recommend starting with a Golden Seed. Your primary stats are Intelligence. Your secondary stats are Vigor and Mind. This build is highly focused on the Sword of Night and Flame for your main armament, along with the Carian Regal Scepter and Frenzied Flame Seal. Any armor set is fine to use however I suggest using the Night's Cavalry Helm, Night's Cavalry Armor, Lionel's Greaves, and Lionel's Gauntlets. Talismans for this build are the Magic Scorpion Charm, Great-Jar's Arsenal, Assassin's Cerulean Dagger and Shard of Alexander. For your spells equip Rancor, Ancient Death Rancor, Golden Vow, and Terra Magica.