To convert to Witchcraft (also known as Wicca)one must follow the procedure below.
STEP 1: Make peace with yourself about it. Going into this is a choice that must not be taken lightly. By becoming a Witch, one must take on certain responsibilities, rather than accepting that someone else is going to do it for you. You will be required to teach all who appear worthy and are bound to keep the Old Religion alive.
STEP 2: Drop any association with any other religion, but do not deny the truths they present. Remember that truth doesn't discriminate religiously. Keep your mind open, especially when you begin on the occult path. One cannot be a Witch and a Christian, because Witches don't believe in Hell or the Devil, or that Jesus was the son of God.
STEP 3: Begin your studies. This is probably the most important part. You will never stop studying if you decide that you are truly a Witch. A period of a year and a day is required before you can successfully be initiated into a coven, and it is highly recommended that solitary Witches (ones who work without a coven) also study for a year and a day
STEP 4: Decide which god/desses you will serve. Pick the one(s) that ring true with your soul. Witchcraft is NOT EVER Atheistic, but the process of magick is. Decide why you are dedicating yourself to this god and research them completely and extensively.
Most traditions will require that you also pick a totem animal too. Record all findings in a Book of Shadows
STEP 5: Fashion your tools from all natural materials, except in the case of the Athame (a short black handled dagger used to direct energy) you will probably have to buy this unless you just happen to be a blacksmith. Sew a ritual robe together. It should be dark colored, black is the best, because it is dull and wont distract the other you may work with, and it draws energy from your surroundings, making your spells that much stronger.
STEP 6: Begin a Book of Shadows. It is a Book that you write all of your studies in while on your journey through witchcraft. The cover must be black, and you must never EVER let anyone read it. Imagine it as your soul, Would you want someone to just flip through your soul for fun? It must be handwritten in your handwriting. On the First page, write this charm while you are in a magickal circle:
These words herein are mine alone
Fashioned deep inside my bones
Each picture, thought, and word you see
Are all reflections of what is me
Beware the urge to take this book
And read it in some private nook
For its magick isn't blind,
And I will know you've touched whats mine. Write everything you learn about Witchcraft in it, especially during your first year and a day, while you are yet uninitiated.
Step 7: After your studies of a year and a day, try to find a seasoned Witch to initiate you into the Old Religion. If one cannot be found, there are many liturgies out there for self initiation. Most of them take time, and require a period of meditation (which BTW never goes away either.) With Witchcraft, don't rush things. That is how most people decide that it is fake, or that it is evil. they rush and don't take into consideration their actions.
You are officially a Witch after this procedure is finished, and remember that if you rush things, you will not actually be a Witch, but a dabbler.
Being a Witch is not about casting spells, or making magick. It is about trying to improve yourself, and trying to better the world around you. Casting spells that hurt or harm someone is a big no no and will only backfire on you. Be sure this is what you want, because if you go through with this procedure you can not leave for a year and a day.
The answer given above is mainly concerned with the neo-pagan spiritual pathway known as Wicca.
In truth, there is no need to "convert" to Paganism, since Paganism is simply an umbrella term for nature-based beliefs and practises.
A Pagan is not necessarily a Witch, and recognising or discovering one's Pagan beliefs, does not require an interest in or involvement with Witchcraft, or with Wicca.