If I have 2 things that trigger at the same time, how do I determine which goes on the stack first?

When multiple triggered abilities occur simultaneously, they are placed on the chain according to the following rules:

1. Turn Order: If multiple players control triggered abilities that trigger simultaneously, the Turn Player adds their triggers to the chain first, followed by the other players in turn order.
2. Controller's Choice: Within the set of triggers controlled by a single player, that player chooses the order in which their own triggers are placed on the chain.

According to Rule 376.3.b and 376.3.b.1:

• "If more than one Triggered Ability is Triggered simultaneously, then the player that controls the Abilities selects the order to place them on the Chain."
• "If multiple players separately control Triggered Abilities that are Triggered simultaneously, then starting with the Turn Player and proceeding in Turn Order, each player orders their Triggered Abilities on the Chain."

Important Note on Resolution: Because the chain resolves Last-In-First-Out (LIFO), the triggers added to the chain last will resolve first. When you are ordering your own triggers, you should place them on the chain in the inverse order of how you want them to resolve.