For an explorer? Order of do things is to upgrade your FSD first, and it's one of the few exceptions where I'll say "finish the G5" no matter what, because every inch counts when it comes to jump range in an explorer build, at least as far as I'm concerned. Mostly you're not getting enough out of your last three or four rolls at G5 to make it worth the hassle of having to find the mats again, but when it comes to range on an explorer... That's a different matter. Not that you can't get there slower, but there are places that you simply can't plot a route to without taking a major detour if your range is too low.
As to the rest, for an explorer don't worry too much about engineering at first, although investing the tiny amount of time getting the Guardian FSD booster is more than worth it. 10 extra LYs for a size 5 slot is great.
As to the rest, my bit of advice as somebody who started out in a ship with only the most basic of engineering and wanting to go see stuff (it's cool):
As to rates in an explo ship, only worry about D and A rates. D is decent performance, low weight (more range), and affordable price. A is superb performance, a little bit more weight and high price. As a beginning explorer, I'd say only A rate your fuel scoop and your FSD. The rest is fine D-rated, unless you want to try your hand at high G planets, in which case A rated thrusters help a lot. Again, I'm trying to stay out of engineering at this point, as that is a whole different matter.
Flying? Always, ALWAYS scoop when you're at a star that will let you. Don't let the streamers who think it's cool to let their tank run almost empty before they scoop because it's "faster" that way fool you. It isn't. Scooping x tons of fuel takes the exact same amount of time whether you're doing it in one stop or fifteen, total. And if you let your tank run almost dry thinking that you'll find a scoopable star soon, be prepared to call the fuel rats when you realize that your nav computer assumed that you weren't a streamer and remembered to top off before you hit a streak of five dwarf stars in a row.
On that note, if your fuel is running close or below one half, check your GalMap. As a matter of fact, do that regularly. It will keep your route updated and it will mark stars that are "last chance to scoop" with an icon. Don't ignore that. If you're in doubt, don't be afraid to plot a pit stop at a scoopable star along your route in there either.
Shields? Don't do ANYTHING without a "bumper shield", no matter how much you'd like to use that slot for something else, especially not in an explorer. I don't care if you're Starbuck, at some point you're going to bump into something and be grateful that your shield, even if it's just a D-rated bumper, took the blow. Shields regenerate for free, hulls don't. And if you're out in the black, well, it's usually a long way to the nearest shop.
Just a few pointers. Fly safe, CMDR! o7
