Hi
Good advice already given by other posters.
My suggestion would be to pick a system that is high tech. with a large population. Use that system as a 'base' and get allied with all the factions at the main space station by doing trade and courier missions for them.
Aim to engineer your ship for speed, this will allow you to survive interdictions by pirates etc.
Gradually build up enough credits to either add larger cargo bays to your ship, and / or accumulate enough credits to change your ship/s for something bigger.
If you stick with that system / space station missions trade or otherwise will gradually offer you more well paid missions. You will also get to know what immediate systems in your area of space have various commodities for trade, for example:- High tech starports usually need metals (gold etc.) and minerals (indite, bertrandite etc.) plus other stuff.
So you'll gradually have a good idea of what commodities to trade with for the nearby systems. By trading with nearby systems your ranking will have a positive effect with those systems factions as well, so it all has a knock on effect, your rank increases and missions pay more.
Stacking missions from one starport to another accumulates more credits per trip, but this will attract more interdictions from pirates (hence my advice on a speedy ship).
At some point you will need to seek out the various Engineers to upgrade your modules for speed, jump range and weaponry (if you decide to go on the offensive missions etc.)
How you progress is up to you, as others have stated, you can do exploration, trade, combat, mining plus other various means to progress through the game.
Good Luck!
Jack