align bootstrap form

With your code it's 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-2 in each label or you can make use of 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>

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 so

<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>

I hope it serves you

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

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

I hope it works for you.