Should I get an attorney if I am facing simple assault charges and why?

In December, I went to visit my fiancé who is in the Air Force.. We got into an argument that quickly escalated, and I wound up slapping him in the face. He had been drinking. The police were called, and we both were arrested and charged with simple assault. We did not press charges against each other, but I still have to go to court. I am thinking about pleading not guilty but I am unsure if that is the best thing to do. What could the possible outcome be if I am convicted? Should I get a lawyer or can I defend myself? Is this case going to be treated differently because the alleged assault happened on an Air Force base? Thank you in advance.