Step one: get trained. Take the NRA handgun course or any sanctioned licensing course in your state. If you don't know where to start, just ask at your local gun shop. $100 is really not enough to get a quality gun, but you might pick up a decent small .38 revolver. Then, practice, practice at your local range. Above all....get the training before anything else, or your efforts at self defense will literally backfire. And it goes without saying, get advice about local carry laws in your state and don't do anything illegal, unless you care to spend a few years showering in the county jail.