The American Academy of Pediatrics has a section on their website directed at parents that looks at what immunizations are needed for different age groups and considers questions parents may have about the safety of vaccinations.
All 50 states require that students be immunized and will not permit children without immunizations to attend school, although there are exemptions available for medical and religious reasons (which vary from state to state). Check out the requirements in your state for immunizations and speak with your child's pediatrician to make sure that your child also receives non-mandatory vaccines, such as a flu shot, when appropriate.
The Centers for Disease Control has created a list of childhood diseases that can be prevented by vaccination. Parents who question why their child needs to be vaccinated should find this important reading.