Can I start using my new deck right away

Yes — you can use it today. Two minutes of setup is all it needs:

1. Count it. A full RWS deck is 78 cards: 22 Major Arcana + 56 minors in four suits. If yours comes up short, find the missing card before the first spread — a gap mid-draw is the only thing that really ruins a reading.
2. Look at one card before you "interpret" anything. Pick the Fool, note what's on the ground, the bag, the posture. Build your eye from pictures, not from a memorized definition file.

Nothing else is required. There's no consecration ritual, no "let it sit in a box" rule — you can shuffle and pull the moment it's in your hands. If your deck is RWS-based (the standard imagery Rider–Waite popularized) I can read it directly, card by card, here.

One caveat worth knowing: the cards I draw and read for you come from the standard deck built into this skill, not from your physical one. So if you want to learn your own deck's imagery, the honest method is: pull a card, describe the picture out loud in one sentence, and I'll tell you what position and suit it is and what that seat is doing. That way the deck in your hands is the one you actually learn.

Want a practice pull on that — one card from the built-in deck, and you describe it back to me?