If the opponent enters my battlefield with a unit that says "when I attack" but in the same battlefield I have a unit that says "when I defend", which one procs first?

The "When I defend" trigger resolves first — even though the "When I attack" trigger goes on the chain first.

Here's why: when combat opens, the attacker (who applied Contested and has Focus) puts their "when I attack" triggers on the chain first, and the defender puts "when I defend" triggers on last. Since the chain resolves last-in-first-out, the defender's trigger resolves first, then the attacker's.

So: "procs" (goes on chain) = attacker first; "resolves" = defender first. (Rule 464.2.e.1, Rule 340.1)