How can I evict a live in girlfriend

The same way you would evict anyone. First you have to be either the owner or sole leaseholder of the property. If she is a part owner or signed on the lease, then you can't evict.

Once you pass that hurdle, you have to determine her status. Did she sign a lease with you? If not (and I'll assume not), then she is a month to month tenant.

Check with your local housing court for the exact eviction process in your area.