Since you said you are a Mara, you are only going to be a DPS. There was an earlier reply that gave you a priority on what attributes to focus on and why. Invest time searching the forums and Google to find out how to run your character. Things to know are stance, how to allocate points in your Advanced Class Tree and why (aka builds), articles on abilities rotation. Make sure your armour has those attributes given (str, end, pwr, crit, etc) and you keep them up to date with your level. Also, too many people handicap themselves by not equipping relics, implants and earpieces. Those give huge boosts, just keep them somewhat equal to your level. Some items are moddable, so same thing with Barrels, crystals, mods, enhancements, armor, augments- keep them updated and relevant to your character. Putting mods with cunning on a Mara won't be doing you too much good..
Know what CCs are and how to use them, especially when you are in a group.
Don't forget you companions. As you get a new one as your story progresses; find out what their purpose is( heals, damage, tank) and equip them and set their stance accordingly. Keep their gear updated.
This all costs you credits of course, so know what to prioritize spending on for updating items.
Know how to set up your quickbars, including binding.
Know how to use stims, adrenals, even medpacks and your relics.
Find complementary crew gathering and missions skills.
There are a lot of resources scattered around the internet, but you'll need to invest a little time to finding them, reading them, understanding them, practicing them, and finally refining them based on your own comfort and circumstances (PVP vs PVE, flashpoints, Raids, specific role in Guild). There are articles and guides with in-depth info out there for most every class, and there will be reasons given for the why, and even alternatives given.
Not being facetious, but this is important based on how new to this game you are and to MMORPGs overall. SWTOR is my first game in this genre, and I had to learn like I said above. Took a little time, and i'm still finding out new things, but the time spent was well worth it. These forums and sometimes even General chat is a good way to answer specific questions. Joining a Guild helps too if they are willing to help their members.
After a while, and this is only my opinion; roll an alt of another class. This will help reinforce what you had to learn. It helps to keep the game interesting. Also, if you roll a character from each of the four classes on either faction, and progress each character up through the end of Act 2, you get the full range of class buffs.
Ok, getting too wordy, and this is only a synopsis. HTH.