ANDY'S ANSWER: I think I would like to know the exact circumstances of the bullying and how you are defending yourself before I say it's ok. For instance, if a bully trips you, is it ok to take a baseball bat and whack him along side the head? Um, no.
If someone is hitting you, can you grab their arm and secure it behind their back? Can you hook your leg behind their heel and pull them down? Yes.
If someone is calling you names, can you say, "I know I am, but what are you?" Yes. Can you spike their chocolate milk with a bar of Exlax? No.
Defending yourself is fine as long as your focus is stopping them from injuring you, not to injure them. There may be times when the only way to stop a bully at that moment is to make them hurt as well, but I will caution you that as soon as you enter into that territory, you risk being seen as the bully instead of the victim.
My advice is to avoid a known bully whenever possible, hang or walk with friends whenever possible, and document all instances of bullying however possible. (eg take pictures & screenshots, save texts, make notes, etc.) Also make sure that an adult is aware that you are having issues with someone. Report any instances of bullying that you see towards someone else. Standing up for someone else may mean that someone else will stand up for you.