Staking should begin automatically under most circumstances, but if you are experiencing issues check that the following conditions are met:

  1. Chain must be synced, not importing or reindexing
  2. Chain tip’s timestamp must be past a certain time (was many many months ago)
  3. Must have active connections
  4. Wallet must be unlocked (unencrypted wallets are always considered unlocked)
  5. Must have mintable coins (aka, at least 51 confirmations - confirmations can be checked by mousing-over transactions)
  6. Must have more coins than reserve balance (reservebalance default is 0)
  7. Masternode additional data must be synced

You can check the status of these conditions in the command-line interface or using Tools -> Debug Console with the command getstakingstatus

Staking can sometimes take up to 30 minutes to begin working even after conditions are met. If all conditions are met but staking does not begin after some hours, restart the wallet.