To stake ADA you need assets in your ADA Shelley address (minimum of 5 ADA)
To stake ADA on Infinito Wallet, follow these steps:
Step 1: At Home screen, Select “Earn Interest”.
Step 2: Choose ADA Shelley and click “Proceed”.
Step 3: View the information about staking. Then, tap “Stake now”.
Step 4: Select a Validator
Step 5: Enter your Wallet Password. Now your crypto is staked!