A servitor, as I understand it, is a thought form -- which is an extension of one's own energy. There is no "spell" in the sense of a chant to create one, that I know of. Rather, it is a process of visualization.
A servitor is created with a very specific purpose in mind, and it is often advised to destroy it once its purpose is filled. They do require a good bit of energy to maintain, and some believe they can get out of control if they are left to work without instructions.
It may not be as detailed as what is called a tulpa (which is a "whole 'nother level"), but a servitor needs to be created with great care. Its appearance, some say, must be finalized before its creation, along with its full intent. Its creator should (basically) supervise it as it exists, and make sure it sticks to that purpose, and be ready to dismantle it at the slightest deviance, or else face risks of it getting out of control.
It would have basic autonomy, but only based on specific instructions. It cannot make its own decisions.
There is one school of thought which says that a servitor is basically manifesting another personality within one's mind, sharing the mage's mind space with the creation (as it does use its creator's energy, this makes sense in a way).
In stead of just doing a spell and hoping, I would rather advise you to find some decent study recommendations, and research exactly what it is you want to make, and for what purpose.
An easy example would be (and this is purely hypothetical) a witch creates a servitor to deflect negative energies while performing a very involved ritual. After the ritual, when all is said and done, the witch will typically destroy the servitor as its purpose is fulfilled.