Add Form Validation

Add input validation to my form. I want to check user data before accepting it.

Framework: [Express, Flask, Django, FastAPI, React, Vue, etc.]
Validation library: [Zod, Yup, Pydantic, Joi, or suggest one]

My form has these fields:
- [email, password, username, age, phone, etc.]

For each field:
1. What rules should it follow (required, format, length)
2. Clear error messages users will understand
3. Show validation both on the frontend (immediate feedback) AND backend (security)

I'm learning, so explain why each validation rule matters and what could go wrong without it.