First off, welcome to the forum!
The topic of when and how to bring up the topic of polyamory to a prospective date (or even a friend) is one that gets debated often. As usual in this poly world it seems that there is no one really good answer, and you have to work out what is right for you (both something that you can morally live with and also gives you the desired result).
My current OSO I met at a poly gathering - so that was kind of a moot point, since we were both poly.
Before that I kind of brought up the subject not as a "I have an SO but would like to date you, I'm poly", but more along the lines of while talking to them about life and ourselves, mention the term and ask them if they have heard of it. Some have, some haven't, and describing it as "responsible non-monogamy" and something very different from cheating is a good start. Sometimes they will (metaphorically) run - and some feel that this is a shame - it doesn't bother me, though because if they run at just that, then chances are trying to make a poly relationship with them work would be, well, a lot of work.
Sometimes they will ask you more, and I recommend some good online resources to them which they can read and learn and then come back to you with questions, if they want. The good things about this are that they can see that this isn't just something you have made up in order to get to do the horizontal boogie with them, and it takes the onus off you to explain everything.
The other thing that it's good to have is a good, coherent idea of how you "do poly" - as you will read, there are many, many different styles, and I'd bet that not all will suit you. Discussions between you and your partner about how another person would "fit" with you are vital to have, so that you can communicate this with a prospective date and so that they know where they stand and what they are getting themselves into.
OK, there's just a thumbnail from my perspective. I hope that some of it is useful.
"Listen, or your tongue will make you deaf." - Native American Proverb