All romantic relationships should be based on trust. Both partners have to fully trust one another to create a lasting union. One partner needs to be able to feel that the other will have her back at all times and never lie to her. Therefore, when you break that trust by lying, consequences will result. Your girlfriend may never trust you again, she may ask you for a separation or she may even break up with you for good. If you want her back, you're going to have a hard time rebuilding that trust.
Be 100 percent honest with your girlfriend about why you lied. Don't blame your lie on anyone but the person in the mirror. You are the one who concocted the lie, and you are the one who has to pay the consequences. Use this opportunity to come clean with any other lies you have told your girlfriend in the past. Be as upfront with her as possible. Regardless of her final decision, she'll appreciate your honesty.
Let your girlfriend know that you understand she is hurt and angry at you. Tell her it's alright for her not to trust you again so soon, and that you know it will take time to repair the damage. Assure her you are working on changing yourself and leaving all the lying in the past. Be very sincere and look her straight in the eye so that she knows you are serious about changing your ways. Give her time to recover; she won't just warm up to you again quickly.
Apologize to your girlfriend. Saying sorry for your actions won't be enough on its own, but it's the one word she's looking for above the rest. Apologizing lets her fully know you have realized what you did was wrong, and you are truly remorseful for it.
Make sure to never lie again if your girlfriend takes you back. Remember, she's given you another chance. You probably won't get a third. Make the most of it, and come clean with every issue from here on out.