The key ingredient is fluoride. Mild abrasives remove plaque; detergents foam; flavors make it pleasant.
Practical
Brush twice daily with fluoride toothpaste; spit, don't rinse, to leave fluoride on teeth.
Kids
A rice-grain smear under 3, a pea-sized amount for ages 3–6.
Evidence & sources
Fluoride toothpaste helps prevent tooth decay. Evidence: established. Known: Supported by systematic reviews of trials. Unknown: Young children need only a small amount and should be supervised. Source: US Centers for Disease Control and Prevention, Community Water Fluoridation / About Fluoride (Current revision).