Run truffle develop in one terminal session:
truffle develop
Run tests:
truffle test
Start development server:
npm start
The start command will bring up the page at http://localhost:3000/
Enter a new value in the text field, click Submit or press enter to submit the form. MetaMask should reply with "Transaction confirmed!" If it responds with a nonce error, reset the account via MetaMask -> My Accounts -> Settings -> http://localhost:9545/ -> Reset Account.