Well the first thing you should know is where and how to upgrade. Not every station/outposts have an outfitting store which is where you upgrade your ships. For the best places to find upgrades you want to look in your Galaxymap and filter out every kind of economys exept for "High Tech" and "Industrial" with as large a population you can find/get to. Those system usurely have the best chances for any upgrades you want.
second thing if you want mainly to trade/explore you want a fast ship with high jump range capabillietys, not weapons ect. And for that you want the best PowerDistribution(PD) you can afford and also the best PowerPlant(PP), FrameShiftDrive(FSD), Thrusters. The Lifesupport and Sencors you only upgrade to D-rated because those are the lightest in weight(light weight longer jump distances)
third the internal compartments are the ones where you ofcaause get some cargoracks to make room for trading cargo. Here you'll also deside whether or not you want to have shield on you ship or get even more cargo space.. It depends on your flying skills and what you are going to do with the ship IMO.
Here are 2 sides you can use to make different load out of your ship(s) and you can also see how much it's going to cost you...
http://coriolis.io/
http://www.edshipyard.com/
There are many many different combination so it easier for you to try it out yourself than for everyone to give you there opinion of what best for this and that.
Btw it doesn't cost you anything to invest your credit in upgrades as you'll get 100% the cost of any module back when either switching or just selling them again. Only when buying ships there is a 10% lost of value...
Also i would get out of the starting system you started in because the station is very very far away from entering point of the system, in fact you shouldn't be using stations further away than ca. 1000 Ls away from entering point as this is the most dangerous point of the game in the beginning due to NPCs interdicting you or at least try to...
If anyone has further thing to mention please feel free to do so as I'm sure I forgot some things....
