Hi mrshy!
The only way to improve your finger's strength, flexibility and speed is to practice. I find that every sleight requires it's own "touch", and the way to develop it is to do it over and over again. Don't give up if something is too hard the first time you try it. Keep working on it and soon you'll see improvements. That being said, I'll second RealityOne in saying that what makes a sleight deceiving isn't the speed with which it's done.
All the resources RealityOne listed are absolutely phenomenal, and I don't really have anything to add, except that if you're really concerned about your fingers' dexterity you could look up finger flourishing or cardistry (
Genesis V1 by Andrei Jikh is great for beginners); however, it really isn't something that you need.
I like to keep two decks, one for practice, one for performing. The one I use for practice is a cheap Bicycle or Bee deck, generally Bicycle Standard with Air-Cushion Finish. Know that you'll go through practice decks at a fast rate, so it doesn't make much sense in getting expensive ones for that purpose. Avoid 100% plastic.
I hope this helps. Welcome to the world of magic
.