Your ship's parts are rated from E to A. E rated modules are the worst, but also the cheapest. D rated modules are the lightest (if you're planning on exploring or trading over long distances, go for these because the lighter your ship, the longer your jump range). B rated modules are the most durable, and A rated modules are the most efficient.
Everything also has a number next to it, denoting it's size, which is very important. Make sure you check and see when upgrading that you don't accidentally downgrade the size of your modules when you're upgrading their class. Trust me, you'll feel like an idiot if you buy an A3 frame shift drive instead of an A5, then wonder why your jump range just dropped to about 3 feet (and yes, I'm speaking from experience). There are instances where you may want to downgrade a modules size to save on weight, but just be careful.
Everything also has a number next to it, denoting it's size, which is very important. Make sure you check and see when upgrading that you don't accidentally downgrade the size of your modules when you're upgrading their class. Trust me, you'll feel like an idiot if you buy an A3 frame shift drive instead of an A5, then wonder why your jump range just dropped to about 3 feet (and yes, I'm speaking from experience). There are instances where you may want to downgrade a modules size to save on weight, but just be careful.