How to start mongo db in ubnuntu ?

You can start MongoDB on Ubuntu by running sudo systemctl start mongod in your terminal. 

Manage MongoDB Service

Start MongoDB: sudo systemctl start mongod
Check Status: sudo systemctl status mongod
Stop MongoDB: sudo systemctl stop mongod
Restart MongoDB: sudo systemctl restart mongod
Enable on Boot: sudo systemctl enable mongod 

For official guidelines and documentation, check the MongoDB Documentation for Ubuntu.