What is the year and model of your car.
To start with I would get your coolant level checked. But Only check it when it's cold never hot or you could burn your self.
Your hand book should tell you what the level should be.
If you have lost the coolant when you refill it you may need to bleed the system depending on your model of mini. You must use the correct coolant as the boiling points vary the minis run hot and high pressure
If your coolant level is correct depending on your model it may be a bad thermostat or water pump
Post back and someone will be able to help out :-)
Sorry I can't be of more help