Focus on losing weight and hopefully some muscle accrual will occur at the same time. Establishing a diet that is composed of foods you will stick to long term is much more important than finding some fad diet on the Internet to try. You are not starting a new diet that you use until you reach your goal, you are making a life change. Meals should be built around protein and veggies and carbs as your filler.
Read read read. Once in the gym you need to establish a workout program. Then either the trainer at the gym will show you how to use the equipment or you can YouTube the exercises in the program you are going to use so you know what the proper form is. It’s very important to start with LOW weights until you understand form to avoid injury. Leave your ego at the door. You will think you look weak and that’s ok.
Once you have a program established and have found solid food choices you want to stick to it for a few weeks and see what results you have and then make changes from there.