I have ford fiesta 2013 and i would like to ask how to repair the coolant container because i always fill it into maximum level and after using the car its always leaking and the level is always in the below minimum. Someone please explain what to do. 

Can you definitely see coolant dripping from the reservoir? From what you've said you seem to assume it's the res that's leaking and not another fitting.

You have to really make sure you keep an eye on this. If you have a 1.0L ecoboost, overheating is very common and has resulted in a few head gasket failures etc. Hopefully your coolant loss is minimal and coming from a radiator hose or something, however if you can't find any leaks then perhaps it's the head gasket.

They can also have problem leaks from the water pump gasket. Coolant tanks can have microcracks which leak when hot (Ford changed mine under warranty for this). Sometimes the coolant cap can separate internally then leak steam under pressure. Don't try to repair the tank if that is what it is - just get a new replacement. They don't handle the heat cycles particularly well.