You might have learned to breathe in while you lift and breathe out when you lower the weight. You might have learned to "suck in your belly". You might have learned all kinds of things when it comes to breathing and how to use your "core", "abs", or whatever-you've-learned-to-call-it. Whatever you've learned, place that knowledge in a box and put that box in the closet. Here's how you do it when you lift.
Stand in front of a mirror and take a deep breath. If you're like nearly any adult I've ever helped, you'll notice how your chest puffs out and your shoulders come up. This is a habit we're going to get rid of.
Now try to breathe in such that your shoulders don't come up and your belly expands instead of your chest. I know, you won't look as hot on the beach when all of a sudden you get a beer belly instead of a big chest but that's how it's going to be from now on, at least when you lift.
If you have trouble with this, you're not alone, even though you used to breathe like this all the time. No really, look at any small child - that's how they breathe! You just lost it with age, which is a shame because it made you weaker and probably more stressed as well (pop quiz: how does your breathing look when you're highly stressed?).
Try it a few more times. Still can't get a hang of it? Try laying on your back. Place a fairly light object on top of your belly. Now breathe in a way that the object moves up and down, that's usually a lot easier.
Once you can do it lying down, try to stand up and breathe the same way.
Once you get a hang of the breathing it's time to apply tension to your midsection. This serves at least three purposes: first it instantly makes you stronger in every lift (yes, really), second it protects your lower back, third it will fix a lot of issues you might have been told is due to lack of mobility.
So again, stand up in front of the mirror and breathe in a way such that the belly expands. Keep the air there! Now tighten up your muscle! Not by pushing the belly outwards and certainly not by sucking it in, rather like your body would react instinctively if you got a punch in the gut.
Can't relate to the gut punch? Try this trick: take the air again and hold it, now push your fingers into the sides of your belly. Push them out! Of course while still keeping the air.
How to lift
Now that you've learned to breathe and apply tension properly, we'll have a look at how to apply it.
Consider a squat... Actually do this right now! Stand in front of the mirror again in a squat stance (shoulder width or wider, toes pointing slightly outwards). Breathe in as you've practiced and don't let any air out until I tell you to. Push your fingers into the sides of your belly and push them out - now squat down. As you go down and as you come up you still hold the air and the tension. Only breathe out once you've finished the movement. If you want to do another rep, you take a new breath and tighten up again.
The above applies to all heavy lifts: squats, deadlifts, bench presses, snatches, rows... All of them. Again, it protects your back and makes you stronger. Practice it every set and every rep, even durng warm-up sets. Eventually it will become automatic.