Go to a system (hint: Wu Guinagi + HIP10716), stay there and get allied with the factions there by running simple (bad paying) missions for a few days. It's a thankless grind, but once you are allied with some of them there the good missions will start coming up (like 6-20M tourist missions). The bonus on Wu Guinagi is that most missions go to HIP 10716, and most missions in 10716 go back to Wu Guinagi, which is a 12ly jump. You can keep bouncing between those systems and level up rather quickly.
The data missions pay a few thousand at the start, but as you rep up, they turn into 50-60k each, and when you stack 20 of those, that is 1M per trip. Any ship will do here; eagle, imperial eagle, viper, cobra... data missions need no cargo racks etc so pick the fastest ship to be able to boost away from interdictions and not lose missions.
Once you have 2-3M saved you could get a type6 and outfit it with some passenger cabins and start running the rep missions for slaves/soldiers/passengers and earn even more credits. You can make 2M per leg sometimes, kind of starting a snowball effect by gaining rep and higher paying missions and getting a better ship to take on more diverse missions.
After you get allied the really juicy missions become available, and before you know it you can get a python or conda and start saving up for decent modules.
Tip: Buy all ships and modules at Ray Gateway in Diaguandri, as they have almost every module there and always a 15% discount for modules and ships.
The key is to rank up rep there as fast as possible while you also rank up with the superpower. Once you level up to say Knight for the Empire you will get better missions overall. I ranked up to baron in three days to get the Clipper, but it also gave a lot more missions that pay well as I am allied with all factions in that system. Now I do 20M per hour basically without any of the get-rich-quick exploits that get nerfed at some point. So even legit game play can earn nice credits.
Right now I basically have every ship I want up to the Anaconda, all fully kitted out and have 50-60M in the bank to spend of stuff; when that runs low I do some tourist missions in my Asp and get back to around 60M quite easily and fast.