First, understand that you must have a companion to get any type of skill to craft, gather or run missions so as a general rule, if you have not left your starting planet you are not to this point yet. Also understand that crafting, gathering and mission (CGM) skills are really things your companion is doing for you (even though you can harvest/gather).

You can only pick a total of three skills. Only one can be a crafting skill.
Crafting Skills - You are limited to choosing just one of these. Choosing one that helps your specific class is suggested. So choosing Armstech as a Sith Inquisitor may not be the best move as it doesn't help you directly, but hey, you can still sell the items so it's not terrible, either. Great money-makers are Biochem and Cybertech but they can also be the most difficult to understand.
Gathering Skills - It is wise to choose one that goes with your crafting skill. If you don't care to craft, then choosing at least 2 is a good way to earn money, plus help the crafters in your guild get supplies. Slicing is special though. This does grant some parts, but also money and item drops. I highly suggest getting this if you don't plan on crafting.
Mission Skills - These are missions you send your companions on. If you go into your companion list you will see the mission skill you have chosen. Clicking it, you send your companion to gather things for you. Sometimes specific supplies or even schematics. While on a mission, your companion is unavailable so don't do these around bad guys!