What do when your going out with someone but you like someone else?

It depends on how long you have been dating person 1 and what type of relationship you have with that person. A date or one or two times doesn't make a relationship, but 6 months to a year with someone is pretty serious. If you fall into the first category I would go ahead and date person 2 without saying anything to person 1. You are not married nor serious, but if you fit into category 2 and this is a person who have been seeing for a long time you may want to say something. If you do this is may scuttle the relationship with person 1. I did this one time and the reaction was not good. He completely broke off our relationship. I had misjudged the result and have been sorry ever since. Plus, person 2 didn't work out either and I had lost the man I loved over the whole thing. So, you have to judge what you want. Sometimes the best result is when you do nothing. That too is an option.