First off, you shouldn’t be nervous in a speedo! Everyone else has to wear them, just try and be comfortable in your own skin. If you think about it, you are most likely the only one that is nervous about wearing one.
There are jammers too, thats an alternative, but they all kind of look the same to me personally. Use your insecurity as a drive to maybe lose that extra weight and be a motivator. I was honestly really insecure at first, but being surrounded by everyone else who seemed to be naturally comfortable helped.
I would recommended SWIMMING, SWIMMING, SWIMMING. Also, having a healthier diet is a lot of the problem if you don’t eat so well. In polo, your stamina needs to be high, so if you swim for long distances, it will help. Also, at my school, a lot of the guys had to do strength training, so they did hundreds of push ups, sit ups, and even ran.
Honestly, I don’t think anyone truly reaches their “good enough shape” level, because there is always improvement. When you get to the point when you feel like you can tread water with ease, and swim at various speeds, I would say you’re ready. You have to remember, you’re not going to be playing a full game your first day, so progress is gradual.
And, don’t worry about those guys!!! You’re most likely not going to be the only new guy. Just act comfortable, and slowly get to know your teammates. Don’t be awkward. Come off anon if you’d like to talk more :)