Start with no longer drinking your calories. Stick with water (milk is good, bit dont start drinking a gallon unless you know gomad). Diet coke has no calories and isnt a bad idea on special occasions, but just know that you will get sugar cravings from the shitload of aspartame.
Try to move more (either exercise or implemented in yo
ur daily lifestyle)
If you are eating and know you are not hungry anymore, just stop. If you feel hungry or start craving drink two glasses of water anfd weight a while. If you still feel hungry, try to substitue the bad food with healthier (read less calories and sugar) substitutes.
Voila your first steps to a healthier lifestyle and especially to losing weigt are set.