I cannot emphasize enough the importance of research. Part of the enjoyment of roleplaying stems from interaction with other people and those other people enjoying interacting with your character. If a character is not properly researched it can be jarring to other players. To be fair, I don’t think that you have to know everything about Star Wars in order to roleplay a character in SWTOR, but if you play a specific species, you should have a basic understanding of that culture and why your character does or does not fit in with that culture.
Secondly, simpler is better when it comes to personality and character history. Just as it is when you meet a new person in a social setting, the first time your character is introduced in roleplay he or she is not going to spill his or her entire life story. In fact, it’s more fun for other players sometimes when your character remains a bit of a mystery. Also, if the character’s personality is close to yours, then it’s easier to play, but be mindful that the other characters you’re interacting with might not be a reflection of their players. That said, sometimes it’s easier to play a personality that is quite different than yours because there will be greater separation. My advice: stick to something close to your personality, but vary it slightly so that you have just enough separation so that you don’t build too much of an emotional attachment.