How do you cause a compression nouns beside PEX pipe ?

I've pushed the fitting into the end of the pipe but it is too loose to be watertight, it near some sort of compression ring ?

Answers: There are special compression rings for the size tubing and fitting your using and then within is a compression tool for the different sizes to crimp the rings. (and it's expensive)

Use PEX fittings or properly sized plastic or vinyl compression rings.