Transfer of heat. First you need a hot pan (sprinkle some drops of water on the pan, it should sizzle). Then you add a drop of oil and swirl it around. When the oil forms a striated pattern, it's hot. Then you put the meat in.
The hot oil helps to transfer heat from the bottom of the pan to the meat. You only need a spoonful of oil.
It depends some on technique, and some on what you're cooking. And it depends on your definition of "sticking."
Foods that are high in protein (especially those low in fat) are more prone to sticking. So a really lean white fish, which is almost all protein, will want to stick. Likewise, egg whites can stick. To some extent, almost any food that doesn't have a generous amount of easy-rendering fat will want to stick, but higher protein is more sticky.
To counteract this tendency, you need to compensate by putting some kind of fat (usually butter or oil) into the pan in advance of the food. Most people seem to get the best result by preheating the dry pan some, then adding the oil, letting it get up to temperature, then adding the food.
That's the simple summary, but it can get you pretty far.
There's another thing you might run into as well--high-protein items that stick at first will release on their own if you let them cook long enough at reasonably high temperature. So if you throw a steak dry into a hot pan, it's going to stick at first, but as the Malliard reaction occurs (that nice brown crust we love), the steak will actually release from the pan (or mostly release) of its own accord--usually after 3 minutes or so at high heat. The same is true of most meats, but with delicate ones like fish you're probably better off providing more fat/oil than counting on an easy release as the proteins denature.
Finally, what's your definition of "sticking?" If your food literally sticks to the pan, then the technique I gave in paragraph 3 above should help you out. If you are able to move your food around, but find that little bits stick to the pan and turn brown, that's to be expected. That's called "fond" and most people try to retrieve those delicious browned bits by deglazing (putting in a liquid and stirring/scraping as it boils down) and making a pan sauce. And even if you don't want to make a pan sauce, you can clean those stuck brown bits off by putting just a little water in the pan and bringing it to a boil. Scrape the pan with a wooden spatula (or even a regular spatula if you're gentle) while the water's boiling and stuff should come right up.
Best Answer
You shouldn't heat a non stick pan without the oil, because it is too easy to overheat it and ruin the coating.
So, I put oil inside first, and tell by the state of the oil. It smells, looks and flows different when it is hot enough. You can also learn to judge if there is sufficient heat when you hold your palm at a given distance above the pan, but in my experience, you learn to judge by the oil before you have gotten good enough to keep the same distance and to distinguish "enough" from "not enough" heat.
If you don't have enough experience to know when the oil is ready, you have to do it repeatedly until you learn it. It takes a few less-than-great meals if you do it purely by trial and error, and goes better if you use an infrared thermometer. You can also try floating cubes of stale white bread and watch their browning behavior, but I find this method too involved logistically - you have to have the bread, cut it, then deal with messy pieces of oily bread, and end up with slowly charring crumbles in your pan.