I just got to 2nd Lt. with 2 stars in ranked. I'm finally pushing past the 2nd Lt. roadblock. If you aren't aware, 2nd Lt. is where the real ranked begins and you start facing people who actually know how to play this game.
Anyways, my advice is that you should train up with 300's until you build up confidence in how well you manage your boost meter and how well you can dodge. Always be step diving and be VERY careful to not overheat unless you can get away with it. Keep in mind that as a 300, you don't want to be too aggressive or else you will find yourself out of boost meter so you should try to play conservatively until you see an opening.
So you first want to focus on boost meter management and learn how to not overheat. Next, you want to focus on dodging and staying alive with your meter. Then, you want to focus on being opportunistic with your boost meter to press for an attack if you get a chance and keep the attack going for as long as possible. Boost is everything in this game and you need to learn how to manage it well.
You can also consider switching suits and try some good beginner suits for 300s like Mk-II (AEUG) and Blitz. There's also cool alternatives like Gouf Custom or Ez8 that are mostly ground based and don't necessarily require good boost management as much as other units do. There's also units like Exia that have automatic zundas (three hits to knockdown enemies) to make it easier to manage your boost.
If you want to start with higher grade units, then step up to a 400 so you can play a back role to support a 500 front role player. If you become really confident and can aggro both enemies at the same time and manage your boost meter and dodge, then step up to a 500. The competition standard for 2 man teams are 500/400 because they have the most boost meter to allow for the most dodging and attacking, and that there's at least one spare life left if both members die.
(edit: Though sometimes as a 400, you find yourself as the highest grade on your team, so it falls onto you to go act as the front. it's a good opportunity to train for a 500 grade mentality.)
Of course, there's nothing stopping you from experimenting with 400's and 500's in casual while you get better, but in a serious setting, a bad 400 or 500 are far more detrimental than a bad 300.
Some good 400's for beginners are Freedom and GP-03 and some good 500's for beginners are Sinanju and Nu Gundam.
It also helps to have a general idea of how much boost each grade has. These are my approximations:
500: 3-4 boost hops before step diving without overheating
400: ~3
300: 2-3
200: ~2
(edit: Also, if you're feeling confident, you can change to a 200. Even though they have less boost and are weaker, in the hands of a good player, they are incredibly cost efficient. You do see a few 200's in ranked, but they're only played by very confident players. You rarely ever see them in a competitive setting. If you want to try a 200, I recommend Marasai.)
