Look for an item of hardware called "SharkBite". SharkBites come in an entire array of combinations to fit just about everything. They work with PEX also as there is a small sleeve inside that slides up the PEX pipe to make it solid and not soft. Look for Shark Bites.
Click here. Watch the video.
Lowe's, Home Depot, Menards, Ace Hardware, pretty much any hardware, plumbing, or home improvement store that sells plumbing has them. Find the one that fits your need on the web site above and then find it at one of the hardware stores.
Last week I installed a new bathtub spigot on my mothers bathtub. The plumbing was 3/8 inch copper tubing from 1970. I wanted to tap into it and come off with CPVC 1/2 inch. I found a SharkBite at Home Depot that worked. Snapped it right on, and worked perfect!