To earn money buy products when they are blue, sell when red. Blue means low price, red high price.
The most useful items are the emergency restore kit which can fully restore your ship, and the cloak which allows you to avoid fights. You'll need a scanner of some sort for some missions.
The mining drones are useless as of the current version. Scanning planets is unprofitable. The financial theft item is useful for stealth, but not profitable for money.
Your main priority in upgrading should be expanded navigation and trade. That will give you a surplus of money for buying items, and allow you to level up faster.
Early game, killing pirates is the fastest way to level. Late game trading is the fastest way to levle.