HELP !

Ok so my goal for this summer is to be healthy and to loose my belly fat. I want to start a change in my life and I believe that me loosing my belly fat would make me happy, more confident and comfortable, but I don’t know how to start. Today I literally ate junk food. I also smoke weed and drink a little so I don’t know how this is all going to work out. I really want to get a gym membership which is going to be $10 a month. I really believe I can make this happen I just need to make a plan for it ! Any ideas on what I should do ?