Here are some tips to help you get some early money.
Crew Skills:
Slicing - This is the number one way to make easy money. Don't waste any on sending your companions on missions, unless you need to grind out a few levels. Instead, just pick up the lock-boxes you find around the world. It is essentially free money.
Any 2 other gathering skills (e.g. Archeology, Scavenging or Bioanalysis) - Collect the mats from around the world (again, avoid the crew missions unless you have to) and sell everything you collect on the GTN. I recommend Scavenging and Bioanalysis as strong droids and beasts can be scanned for materials.
Don't buy any gear, use your planet commendations to gear yourself up, and mission rewards to gear your companion. Focus on gearing only one companion, the one you use the most.
Play the GTN. Post your items for the average price, not the lowest, unless you want the credits ASAP. Be patient. Not everything will sell right away, but as long as you offer it for a fair price, and it is in demand (crafting materials usually are) people will buy it. Keep an eye out for items that are under-priced and buy them up to re-list them.
Do as many quests as possible, especially heroics. Heroics are repeatable once a day, so are easily farmed.
Any green items that you pick up and don't need, just vendor. They very rarely sell on the GTN, and can usually sell for a decent price. Blues and Purples can go up on the GTN.