On the Nav map, cycle around the various suns to find corporate-specific bases. In the starting Hoffer's system, you can find Mass and SOLAN facilities pretty close to your base. Maas has some sort of mining operationg going on so you'll find a lot of transports carrying cheap ores. I think there's also an exotics processor in the system, which gets you better ores. SOLAN carries a fair variety, with a wide range of value. Maas tends to escort more heavily than the rest so be choosy about pirating those.
You can get yourself all blowed up in short order if you attack them near a corporate base. What I'll do is find a base that loads freighters with lucrative goods (the exotics processor, for instance) and just hang around. Use Y to target the components / cargo on the vessel (you have to be within 5 clicks to use this feature, until later in the game) and cycle through to see what goodies are carried. Once you find a decent convoy with a light escort, use F7 to form up with that target. When the convoy goes LDSI to the LaGrange point, you'll tag along with them, away from the corporate base and fighter cover; this improves the odds greatly early in the game.
Another thing that improves the odds is the remote missile. It's slow but packs a powerful punch so you can use it to wipe out at least a couple escorts before a fight, or cause enough damage that the freighter will drop the goods right away. Or accidentally blow the whole thing up before it drops cargo - you should play around with those remote missiles a bit to learn their speed, blast radius, and power.
The larger the cargo ship the more damage you have to do before they drop the goods, so you may have to stick with those little flatbeds and the 8-pod hauler until you get bigger weapons. The flatbeds will drop cargo almost right away and usually have a light escort, so they're a decent place to start. The larger vessels may simply enter the LaGrange point before you've caused enough damage; meanwhile, you've ticked off the escort and they attack you while the freighter gets away partly damaged. I guess you could try F7 again to follow it through to the next point...
BTW, there are some nice mods out there that will improve gameplay in small ways. An important one for me is Custom Jafs:
http://www.i-war2.com/downloads/custom_jafs.zip. It lets you assign a larger cargo vessel for Jafs so you don't have to go through so many loading cycles when you take out some larger convoys, you have 40+ pods floating around, and Jafs comes wandering in with that little 6-pod flatbed. I also liked the walkthrough mod, which puts the walkthrough (it isn't a complete spoiler) section into the encyclopedia.
I think I may have to reinstall this thing (hard drive died) and give it yet another playthrough. Absolutely loved this game.