The process of becoming a witch doesn't happen overnight. It is a life change, a new path upon the journey of your life. It takes consideration, study and work. If you have previously followed a mainstream religion, you may have things that take time to let go, and new things that take time to absorb. I have heard many people say it is often hard, coming from a life of Christianity, to feel comfortable praying to the Goddess. All new things take time, but if you are serious upon this path, you will find your way. The Gods call their own home to them.
No matter how you have came about finding the Old Religion, here you are. So where do you go? To the book store. For a novice, books are like the air you breathe. You must have them, or access to them in some way. If you cannot afford, or do not feel safe having books on the Craft, the internet is the next best place.
In both books and on the internet you will find a wealth of knowledge that will help guide you upon your new path. Of course, as with anything else, there is good information and bad information. Avoid any kind of book, or internet site, that speaks of controlling another person in any way, harming them, doing love spells on a specific person, or tells you to chant in latin, even though you have no idea what you are saying (yes, I have seen sites like that). These books/sites will not fulfill your need for knowledge in the Craft and will only serve to confuse you.
Once you have read a variety of books and feel called to this path, the next step is to find a teacher. If you have access to a teacher, in my opinion this is the best course of action. A teacher or a coven can often be found if there is a new age book store in your community. Also, the Witches Voice is a site that offers networking in every state. It has grown extremely large over the past few years and is a valuable resource in the Craft community.
Having a mentor can offer so much to you when you are beginning. There will be things you come across that you have a hard time understanding and need clarification. If you have a teacher, they are just a phone call or email away. If you do not, you must try to decifer things on your own, and may not come to the correct end on them. If you do not have a teacher, again, the internet is the next best place to look.
If you are only looking for a 'how to' on casting spells, then the Craft is not for you. Witchcraft is a serious spiritual path, in which magick is performed, but is secondary to the religion itself. I would suggest you look to ceremonial magick for that.
A couple of things need to be said about beginning this path, in light of recent attitudes about the Craft. Here lately it seems that you have a people who, after reading a few books, feel as if they can call themselves a master of the Art. They throw on a title like Lady/Lord, or HP/s, add some black clothes, a pentacle the size of a hubcap, and they are ready to go. This is not what the Craft is about. If you have spent years following a particular path, have worked hard for the spiritual lessons that have been presented to you, and through this have attained the title and rank, then by all means use it. But think of how you would feel if, after all that, you have a newbie with 6 months and 5 books unde their belt walking about calling themselves Lady Starry Ski or Lord Thunderbutt. It is very offensive. Just like your parents told you when you were growing up (or maybe you still are) 'don't rush things, it will all come to you in the end, and be sweeter for the waiting'. This is true with the Craft. Using titles, putting on airs, and in general acting high and mighty are not going to make you any more spiritual. That is what this path is about. What it will do is alienate you from people whom you may actually want to meet and get to know!
All of this being said the way to become a witch is through study and dedication. Gather all of the information you can. Find the best teacher possible. Read whatever you can get your hands on. Go outside in nature and commune with the Goddess and God. Listen to the trees and the wind and the rush of the water, for this is the witch's world.