I'm going to tell my girl I don't want to go out with her tomorrow night so that I don't get caught going out with someone else
Why are you lying to her? What will you do if you get caught?
I will just lie about it.
Cheating on your gf by going out with someone else is lying to her. If you want to see someone else then breakup with your girlfriend for good, but don't lie to her.
If I get caught out that's not my fault.
It will be your fault and you will be ruining your relationship with your girlfriend if you go out with someone else, and you should think twice about cheating on her as you may end up getting a reputation of being a cheater, and I think you do not want that association.