Pick up archaeology, bioanalysis, and scavenging. Scan every node you run across while leveling up, including animals and droids that you kill. Sell all the mats you gather on the GTN. Set the prices a little bit less than whatever is the lowest unit price when you post them, and your stuff will sell quick every time.
The reason this works out so well is that even low level mats sell very nicely to lazy crafters who don't like to farm, and the mats you gather cost you nothing in credits and almost nothing in time to get, so whatever you sell them for is pure profit. You also will not waste any credits leveling up your crew skills or reverse engineering patterns, as you have to do with the crafting and mission skills.
And once you hit 50, you will have a nicely maxed out farm toon that can generate tons of mats, and a steady income, to fund your other alts that you decide to take crafting skills on.
make money crafting, but you will ALWAYS
make money gathering.
If you check prices on GTN, you can often find items listed much lower than the competition.
I like to do this with things I'm already selling since the prices are fresh in my mind.