Click here for the text version.
Most witches follow a rule or tenet of Harm None. This simply means that one is free to do what one wants for as long as no one is harmed including oneself. It is also considered a violation of the Rede to cast any spells that goes against the will of the person for whom the spell is being cast. In short, most witches will not turn you into a frog unless you give them permission to do so <hee-hee>.
Besides, there is also a Threefold Law that summarily means that whatever you do shall come back to you multiplied by three. Now that should be enough to make one, at the very least, think twice.
Below is a graphic I made for the Rede. You may use it as wallpaper or whatever purpose you feel like using it for. The size is 934 by 768 pixels
Click here for the short version of the Witches' Rede.