What is your age, are you on any meds? If so, what kind.
Unfortunately this is a condition that is present at or before birth. The causes are typically unknown. Many attorneys to sue doctors for what is called a "bad baby outcome." Meaning many people tend to blame the doctors for the condition. This is not necessarily so.
Please see
this website, it is very informational. I could type most of the info here, but it would be a long post without proper credit to the authors.
Please know that you are very lucky at this point in your life that it is only affecting your arm and walking. Many people that I know who were born with CP were in wheelchairs at a very young age (one of whom I know was in one at the age of 2). There are many different levels of the condition from seriously disabled to mildly.
Good luck, and know that you are one of the lucky ones.