A witch isn't something you become with a spell, it's a title that you can choose to take when studying and practicing witchcraft as a practice.
When undertaking magickal practice you can decide whether you want to undertake the negative workings that are associated with "black magick" and if you don't want to undertake anything negative you don't have to.
I would suggest reading:
Starting Out: http://www.spellsofmagic.com/read_post.html?post=71692
The Basics Expanded: http://www.spellsofmagic.com/read_post.html?post=483942
Visualization: http://www.spellsofmagic.com/read_post.html?post=526472
Centering + Grounding: http://www.spellsofmagic.com/read_posts.html?post=829288
How to write a spell: http://www.spellsofmagic.com/read_post.html?post=829629
Troubleshooting Spells: http://www.spellsofmagic.com/read_post.html?post=846276
And also looking into books on the subject, a really good one is: "Before you Cast a Spell" by Carl McColman.
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