The first year of Elite: Dangerous was quite harsh in terms of earning credits. Most combat pilots ane explorers had to resort to trading if they needed to pad their wallet. Over the years, however, other methods such as skimmer missions, passenger missions, and road to riches have dwarfed trading for earning credits.
Thanks to /u/cold-n-sour on /r/EliteTraders, it's been confirmed by yours truly that trading is indeed back and in a big way. You can earn credits to the tune of 12000 credits per ton without breaking a sweat. That's insane. I remember when 5000 credits per ton was considered a gold mine! Here's your path to opulence:
Head over to eddb.io/trade/loops
Input the following filters to search for a lucrative route (NOTE: These routes can change often as supply/demand adjusts)
Put in your desired location to find a route closest to you, click Find
Pick a route a trade
It's that simple. I'm currently using a Type-9 HEAVY (fit here) to do my trading. It's a simple all cargo rack, class 5 shield that can haul 756 tons. If you feel ambitious, you can run a 792 ton shieldless Imperial Cutter. These routes have the potential to net you close to 100 million credits per hour (10 million per 10 minutes)