Everything from the 1 lessons of this topic on a few pages: the key idea of each lesson, the diagram to remember, the points to learn and the traps examiners set. Read it, cover it, test yourself.
A drug is any substance taken into the body that modifies or affects chemical reactions in the body. Antibiotics are drugs used to treat bacterial infections: they kill bacteria (or stop them growing) but they do not affect viruses. Some bacteria are resistant to antibiotics, which reduces how effective antibiotics are. Extended: using antibiotics only when they are essential limits the development of resistant bacteria such as MRSA.
Do not define a drug as "a medicine" or "an illegal substance". The syllabus definition is about effect: any substance taken into the body that modifies or affects chemical reactions in the body.
Antibiotics kill bacteria, not viruses, and they do not "kill germs" in general. Taking an antibiotic for a cold does nothing to the virus but still gives resistant bacteria in the body an advantage.
The antibiotic does not cause the mutation, and bacteria do not "get used to" or become immune to the drug. The resistance mutation arises at random; the antibiotic only kills the bacteria without it, so the resistant ones are left to multiply.