It would be a good idea to speak with, or shadow, a medical assistant if possible and ask a lot of well thought-out questions before deciding to become a medical assistant.
Once you decide, make sure the school you have chosen to go to is an accredited school, and offers certification. Also, make sure that your credits are transferable. When you get out of school, get hands-on experience by working at an internal medical practice or a family practice and strive to become a well-rounded assistant.
Remember to anticipate your patient's (and physician's) needs. Keep a note pad with you at all times to help you remember things, and document everything clearly so your co-workers don't have any trouble understanding what you write on the charts.
A big part of being a good medical assistant is having empathy. Become a good listener, be caring and sensitive to people's moods. Become non-judgmental. Always be professional, and watch out for becoming too casual (it will border on disrespectful if taken too far).
Being a medical assistant is sometimes a stressful, but always a rewarding, career. There is room for growth as well, and if you decide you'd like to move up, you can look at becoming a physician assistant or a nurse.