In basic principle she'll need:-

- her current passport (plus any previous passports)
- 2 passport sized-colour photographs (See UKVI Passport photo requirements)
- proof of her Knowledge of English (unless exempt)
- Tuberculosis (TB) pre-screening certificate
- proof of relationship to the sponsor (for spouse this includes Marriage Certificate)
- proof of compliance with the Financial Requirement (or if exempt then Maintenance Requirement)

Do take time to review the eligibility requirements . Also read through the UKVI Guide to supporting documents: settlement for details of individual examples.

Here's a link to the guidance on the Financial Requirement.