align bootstrap form

You can try as follows:

   <div class="text-center">
<form class="form-horizontal">
  <div class="form-group">
    <label class="col-lg-8 control-label">Nombres</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="nombre" />
    </div>
  </div>

  <div class="form-group">
    <label class="col-lg-8 control-label">Apellidos</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="apellido" />
    </div>
  </div>

  <div class="form-group">
    <label class="col-lg-8 control-label">Correo Electrónico</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="email" />
    </div>
  </div>

  <div class="form-group">
    <label class="col-lg-8 control-label">Teléfono Celular</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="telefono_cel" />
    </div>
  </div>

  <div class="form-group">
    <div class="">
      <button type="submit" class="btn btn-success left">Registrarse</button>
    </div>
  </div>

</form>
</div>

or this way

<div class="row">
<div class="col-lg-4"></div>
<div class="col-lg-4">
<form class="form-horizontal">
  <div class="form-group">
    <label class="col-lg-8 control-label">Nombres</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="nombre" />
    </div>
  </div>

  <div class="form-group">
    <label class="col-lg-8 control-label">Apellidos</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="apellido" />
    </div>
  </div>

  <div class="form-group">
    <label class="col-lg-8 control-label">Correo Electrónico</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="email" />
    </div>
  </div>

  <div class="form-group">
    <label class="col-lg-8 control-label">Teléfono Celular</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="telefono_cel" />
    </div>
  </div>

  <div class="form-group">
    <div class="">
      <button type="submit" class="btn btn-success left">Registrarse</button>
    </div>
  </div>

</form>
</div>
<div class="col-lg-4"></div>
</div>

Hope this can help you

With your code it is just a matter of playing with the Bootstrap grids, so I put all the columns in 4 to give it the centering effect although you can also use the class col-lg-offset-2in each labelor you can use column ordering

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" rel="stylesheet"/>

<form class="form-horizontal" style="margin:0 auto">
  <div class="form-group">
    <label class="col-lg-4 control-label">Nombres</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="nombre" />
    </div>
  </div>

  <div class="form-group">
    <label class="col-lg-4 control-label">Apellidos</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="apellido" />
    </div>
  </div>

  <div class="form-group">
    <label class="col-lg-4 control-label">Correo Electrónico</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="email" />
    </div>
  </div>

  <div class="form-group">
    <label class="col-lg-4 control-label">Teléfono Celular</label>
    <div class="col-lg-4">
      <input type="text" class="form-control" name="telefono_cel" />
    </div>
  </div>

  <div class="form-group">
    <div class="">
      <button type="submit" class="btn btn-success left">Registrarse</button>
    </div>
  </div>

</form>

Try to apply the alignments that bootstrap has, they are very useful, you just have to place the class in the element that you want to align, something like this:

<div class="text-center">Centra el contenido del div</div>  

I hope it works for you.