You're sure that you want to go to school for medical assisting, yet you don't know what they do? Maybe you could see about job shadowing someone who's a medical assistant now. Or just google "medical assistant job description." It makes a difference if you want to be an administrative M.A. or a clinical M.A. In the list of potential job duties for the clinical one, I saw phlebotomy (blood draws), but I didn't see injections for either one. But I looked at the site quickly, and it could depend on your state.
I would hope you have a good basic knowledge of what a medical assistant does before signing up for classes. If you don't know, you need some research beyond Ask.com. And don't worry about injections. They don't hurt you at all. After a while, giving an injection will become routine and you won't think anything of it.
Honestly, if you are fearful or squeamish regarding medical preps, procedures this might not be the place for you.You might be better at the front end, coding, customer service, office. Facing your fears is great, but if your head and stomach aren't cooperating it's not the best choice.