I'm assuming you just need enough to be comfortable leveling crew skills, yes (which is the major credit dump)? I can't tell ya much about cornering a market on the GTN or anything, but in the course of leveling three major toons (though only one is 50 and only one of the others even made it to Act 3, yet), and two side characters, and earning about 600k between them and all having their skills and maxed out crew skills, here's just what I've done:
Tip 1: Space Missions Count: Space is a pretty mindless way to shore up lots of credits. It's not as speedy or direct as other means but it can be a little more engaging than running around farming credit cases. At the very least, the daily quests are useful each day, and if you complete the bonus missions they can earn you some easy creds. For example, one of the first available missions when you get your ship, Cartel Listening Station (I forget its opposite counterpart, but, the space station with satellites), without even upgrading your missile tubes you can destroy the relay at the bottom. That bonus mission nets 3k, plus a good chunk of XP. If nothing else, you can use the commendations on grade 7 and 8 crafting mats, something you normally can't get pre-level cap. An individual piece goes for 50k+, easily.
Tip 2: If leveling multiple characters, you might use a "Mission Mule". For me, this is my Operative. She has Slicing, Underworld Trading, and Diplomacy. I have a Marauder Synthweaver and Sage Cybertech being fed their Underworld Goods from my Op. Having max affection with Kaliyo at Alderaan is making that a breeze. This frees both my Sage and Marauder to take Slicing, and be basically self sufficient for just about anything they need, even extra purple mats and luxury purchases like companion customizations. If you take slicing, it's important to allow yourself to be distracted by the credit boxes. If you're a stealth class, it's even better, you can CC if it's a single guard (and many are "unguarded" to stealthies, whereas other classes need to fight through a horde to get to the box), loot the case, and jump back into stealth immediately and onto the next. My Operative rarely travels mounted, she stealths through territory with humanoids in it and picks up all the cases along the way. Time consuming, but effective, and X-thousand credits richer than if you had taken the road to the next hub.
Addendum to Tip 2: If you only want to make one character right NOW, and mule for later, level the three mission skills with companion gifts. These missions are shorter, cheaper, and will get you that "10k affection by level 30" thing like my Op. has. You can come back and do the metals and gemstones later when you have more affection, and more companions to run the missions.
Tip 3: If you're doing a crafting skill and need a purple component, buy a mission from the GTN or the component itself. Check the prices, of course, but standard affection with no bonuses in your mission skill, odds are pretty strong it's cheaper to dump the 6k right now and guarantee those Ice-Jewels rather than running fifteen, 900 credit missions to get one proc (sometimes it will take a lot). This is more a saving credits than earning credits =P
Tip 4: Check your GTN for a few consecutive days and figure out what sells best from the crafting skills. My server, it's cybertech. I'm sitting on lots of purple mods and armor for leveling that can go for 10k each, not bad for something that, at best, took 600 to make (if purely from missions). And, of course, that is math. Lightsaber crystals might be cooler, but at most, every toon has two. They might have up to 8 armor slots they need to fill as they level, and no amount of commendations will solve that for them (well, they could, but they'd be outdated by the time they had enough =P).
Tip 5: You can become a gathering mule if you see fit. Bio-analysis and Scavenging in particular. This will be especially strong if you do group finder. I've often found myself the only Scavenger, and in a mission like Hammer Station, where -everything- is a droid, that's a lot of mats.
Tip 6: Anything remotely related to Artifice is flooded. Archaeology will not net you a lot, as most of the Artificers are, as well. Hilts and crystals sell pretty poorly unless they are purple grade, and Treasure Hunting has been, in my experience, the worst and stingiest mission skill, even with max affection. Steer clear if money is your goal =P
I hope I managed to help in some way. Welcome to the game! ^_^