Breathing and Embouchure
It should go without saying that a strong, clear, well-supported tone is essential to everything you do with the flute. It starts with how you breathe. This is the key: your abdominal muscles should do the work of breathing - when you inhale, your belly gets bigger; when you exhale, your belly gets smaller. Sit up straight, relax your shoulders, open your chest, and take a few long, deep breaths like that right now. Think of the breath going down into the center of your body, and then flowing up from that point as you exhale. Now try blowing long notes on the flute, letting them rise up from inside you. When you hear people talking about having "good breath support," "breathing from the diaphragm," and so forth, this is what they mean. Your breathing is deep and relaxed and powerful.
So now, direct your air in a focused stream against the far side of your embouchure hole. Simple enough. End of lesson.
Ok, ok, I'll try to be more specific. Bear in mind, though, that there are so many tiny variables that go into a good embouchure -- variables that are different for each flute -- that what follows should only be taken as a general set of guidelines that should be refined through your own practice.
You may find that you prefer having your headjoint rolled in to some extent; many traditional players do. What this does is to bring the far edge of the mouthhole closer to your lips so you can hit it with a good focused stream of air. How far to roll in depends on your tastes, posture, and instrument. If you find that you're unconsciously holding the entire flute in a rolled-in position, or that the flute sounds best if you bend your head down, you probably should experiment with this. On my large-holed blackwood Olwell I generally have the far edge of the embouchure hole lined up with the far edges of the fingerholes. Be careful that you actually roll the headjoint in rather than rolling the bottom sections out - you're changing your embouchure, not your hand position. Check yourself in a mirror. You'll also find the flute can go quite a bit flat when you roll in, so adjust your tuning slide accordingly.
Once you get your breath focused properly, you'll find it takes much less physical effort to play the flute. But even so, you should get used to playing at a good robust volume. The big throaty roar that's favored by Irish players these days comes in part from simply playing at the outer limits of the dynamic range. Don't be shy. When you do play softly, don't let your tone deteriorate - you should be able to use the same focus and breath support as when you're blasting away in full session mode.
One last note: many (if not most) classical players are in the habit of playing everything with a constant vibrato. If you do this, you're going to have to do some reprogramming. Traditional players use vibrato very sparingly in slow tunes, and never in dance tunes. When vibrato is used, it's an entirely different technique.