Right now, we're at a bit of an impasse between plots.
The Miharu right now is pretty much at the cap as far as players go. It's a mainly single-post ship, with JPs in the works. However, it's the most detailed plotship out there. If you prove yourself to be a good writer with well crafted characters, you might be able to sneak on. Maybe.
The Sakura is kind of the flag plotship here. I'm not aware of its player situation, but if you show you can craft a really good character that the GM likes, you might find yourself on there. Many of the veteran players in the SARP "graduated" from the Sakura at one point or another. It also has the most epic plot of any ship.
The Yūgumo is at a crossroads. Right now, there aren't many players active on it, but those that are there are very loyal to it. It might be a chance for you to shine in front of veteran players.
The Goban is my ship. It was slow for a while but is now picking up speed in its current mission (SP only, really). Most of the players there are new, so it might be a good place to stretch your legs and learn the setting. Don't worry, high turnover is expected.
The Alliance is a new Nepleslian plotship that's simply exploding! It's a very hot ship right now, so if you don't mind playing a male character, this might be the place to go. GM Tom can explain it better than I can.
The Yggdrasill is an oddball ship at the moment, run by MM007. It's a civilian vessel, but I think it's going to "take off" as it were pretty soon. It's going to meet the Miharu ICly soon and drop off a character so it can get rolling on its own.
For the other non-plotship boards, you'll have to look them up. There's Nepleslia, Space Pirates and General Roleplay.