How can I give players items that are automatically equipped In Minecraft?

You can use this command to specify with slot to place an item:

/replaceitem entity @a armor.chest minecraft:iron_chestplate{}

And you can do this for the helmet etc.

If you'd like to equip the player with a sword you could do:

/replaceitem entity @a weapon.mainhand minecraft:iron_sword{}

To equip the player with a sword in their hand, i.e. the slot of the Hotbar they are currently using. The same for a shield with:

/replaceitem entity @a weapon.offhand minecraft:shield{}

Important Note

/replaceitem will be replaced with /item in 1.17, but you can easily convert to the new command.

Instead of this:

/replaceitem entity @a armor.chest minecraft:iron_chestplate{}

It should be like this:

/replaceitem entity @a armor.chest replace minecraft:iron_chestplate{}