So what exactly is your background, and why do you need one? A backstory is bsaically the story of your characters life up until this point. My character, for example, was born in a small colony that was destroyed. He then went on to become a jack of all trades. You need a background of some kind, because your character didn't just spring into existence (unless, of course, your character did just spring into existence).
What makes a good background? Well, that's a tough question; the best way to tackle this is probably to ask "What makes a bad background? I'll give you an example : Warlord Khan, a prominent member of the Combine staff, and Members Director has a bad background. In fact, he has a very bad background. Warlord Khan is actually General Rommel, from the Second World War. He was transported in time, and changed his name, so as to fit in. What's bad about this? First, in the Combine, there is no "General Rommel." Nothing that has actually happened happened in the Combine universe. Secondly, using real people as your background is considered bad taste. And finally, the whole idea of being transported in time isn't something that would be encouraged. The idea behind the Combine is that we're all ordinary people in the Combine universe, not superheros who have bizzare and exciting things happen to us all the time.
What else makes a bad background? As I said, your background shouldn't include bizzare stuff that couldn't possibly happen. This is called godmoding. If you were to write a background that included you being raised by an introverted Jedi Master named "Tele-Tubbi," who trained you in all sorts of Jedi techniques, that would be godmoding. If you were to include a brief stint as Emperor of the Universe in your background, that would be godmoding.
Something that I have always found to work is to include a lot of unpleasant stuff in your background. Using my character as an example, his parents were brutally murdered, and he spent his entire life drifting aimlessly, with no goals or friends. He also seems to be a closet alchoholic. An unpleasant background is always more believable than a pleasant one.
One thing that most people might not think of is writing your background to explain your starwars combine skills. For example, Imogen Bailey has a high theft skill, and a low strength. It would make sense for him to have a background as a thief. It would not make sense for him to have spent his entire life as a weight-lifter, or a researcher. He also has fairly good computer operations and piloting skills, so if we're to keep him as an example, we can work up a fairly coherent background based on those skills.
Example 1 : Imogen Bailey was born into extreme poverty in a slum on an out-of-the-way planet. He never knew his father, and his mother died in childbirth. He supported himself by stealing whatever he could, and selling it. One day, he was caught stealing something by a major crimelord, Hi'jarra The Hutt. Hi'jarra recognized Bailey's inborn talents, and taught him some useful skills that would enable him to become a great thief. He learned how to use a computer to break into bank accounts, and he learned how to pilot a freighter, so he'd be able to break into one, and fly off. Eventually, he became Hi'jarra's right-hand man. Once Hi'jarra died, Imogen was named his heir. Looking at all the pain he had caused, all the crimes he had commited, he decided not to accept his inheiritance, and instead decided to make amends. After spending 4 years looking for a cause to which he could devote his life, he joined the Brotherhood of All Worlds.
Example 2 : Imogen Bailey was born a nobleman, the Lord of Dhar'mir. While used to luxury, his father wasted all their money gambling. When his debts became too great, his father took a spacewalk...without a suit. Imogen was forced to steal artwork to survive. His only relaxation from a life of crime was racing custom-built space yachts. When the authorities finally caught up with him, they took everything, and threw him in prison. After his release, Bailey knew he had to make amends, so he joined the Brotherhood of All Worlds in the hopes he could help people.
Example 3 : Imogen Bailey was born into a normal, middle-class Corellian family. As a student, he never excelled at any subject, and ended up going to the Imperial Academy, not having any other real options. Bailey quickly became disillusioned with military life, and dropped out. Not having the funds to return home, he was forced to live in the poor side of a seedy town, working days to earn money, drinking away his money at night. Imogen made friends with a group of people who hung out at the local tavern. They came in one night, overflowing with money, and he learned that they were criminals, making their money preying on civilian transports. He compared what they made with wages, and joined up with them that day. He spent 7 years with them, hacking into flight plans to find out where their prey would be, and flying the ship back to port after killing the crew. He was the only survivor of a disastrous expedition where they were ambushed by patrols. He revels in violence and chaos, and joined up with the Brotherhood because he wanted a chance to kill some force-users.
You may have noticed that all three of these examples include dead or out-of-the-way parents. There is precedent for this, and good reason. Just look at the original Star Wars trilogy. With the exception of Luke and Leia, none of the characters had any living parents. And really, that's a good thing. What would we have thought of Han Solo if he called his mother every month to check in? Would we have respected Darth Vader as much if we saw his father telling him to "Take off the cape, for godsakes, it's hot in here!"? Trust me, parents are a complication that you shouldn't bother with.
I just want to point out one thing Keroban, one of the first RPG trainer in the starwars combine and who made the first draft of this document,i said : Somehow the suffering of a character in the beginning is quite nice. It adds another dimension that nothing else really does.