Pretty much exactly what Katahdin said. As soon as you decide you're going to be a witch and you start seriously looking into what's involved, you can consider yourself a witch. Witchcraft is exactly that, a craft and a practice, so if you're practicing (even so far as learning how), you're a witch, crafting :D
It's important that you know what you're doing before you start trying to cast spells or anything, there's not really any rules per se, but a generally accepted reality is that if you don't know what you're doing and you try to do something to cause someone harm or control them, it'll backfire on you. Plenty of practitioners do perform spells with negative results but they've been around awhile and know how to control their spells; not recommended for first timers.
First steps are to make yourself mentally strong, centred and aware of yourself, which you can achieve through meditation. I know a lot of people don't like it but it's the best fastest way to build your strength. Even a few minutes every day is a few more minutes than you were doing before; like exercise, it builds up.
Next is to think about your goals and analyse them, figuring out what you want to actually do and whether or not it's a good idea. Great early spells to try are shielding and protection, banishing negativity, money attraction, and the first spell I ever did myself was for a stable home life and musical success, both of which, while in early days still I suppose, seem to be doing well. Probably because they harm no one and came from the heart. My money ritual has also been very successful - I only asked for "more than enough for the rest of my life" and that's been solid for about a year :D
If you want any help or advice you can ask me and I'd love to help or at least point you in the right direction, but genuinely the forums have pretty much anything a newcomer could ask for already in the posts, they're a great resource as everything is separated by topic and easy to read.
Congratulations new witch! Enjoy your journey!