Someone I really like is polyamorous, but I’ve always been mono. They have multiple partners, and poly is really new to me; I’m learning a lot about it actually. What I’m wondering is…if I ask this person out, am I expected to date all of those other people too when I’m only interested in just them? And…I’m the kind of person who wants to maybe settle down with someone one day, am I just setting myself up for heartbreak?

No, polyamory doesn’t necessarily mean you need to be in a “closed” relationship with everyone involved. Some people have those as their terms, but many don’t. You’d need to talk to this individual person about whether they expect anyone they date to also date all of their partners. 

If you eventually want to end up with only one person, don’t date a poly person with the hope or expectation that they’ll “come around” and want to settle down with you. Never go into a relationship built on a foundation of denial or hoping that things will change in the future. If you’d be unhappy in a relationship with no long-term potential to become monogamous, this probably isn’t the best call for you.

But, if you aren’t looking for someone to settle down with just yet, if you’d be okay dating someone for fun, for a fling, to indulge a crush, to explore, if you don’t particularly mind that this relationship at this stage in your life won’t end in settling down monogamously, then you may not be setting yourself up for heartbreak. You would need to be clear with this person that you might end up leaving the relationship when you feel it’s time to start actively pursuing a long-term, “settled down” monogamous relationship. They might decide not to date you with those terms, and that’s their right.