First things first.
Figure out how much weight you want to lose and around the time you want to lose it (make sure it’s healthy)
Figure out how many calories a day you need to eat to lose the weight in your goal time.
Find out how many calories you eat in a day right now and how much less you need to eat.
Find out which foods and drinks make up the most calories in your diet.
Research healthy foods that you can interchange with the unhealthy ones. Also, find out where you can purchase the healthy foods instead of the unhealthy ones.
For the gym. Just start out by running the treadmill everyday for 30 min..then figure out what parts of your body to improve and research the exercises needed to improve them.