I'm on the path to doing this rn and honestly it will take time. growth takes time. I think its most important to be patient with yourself throughout it.
But firstly when it comes to physical health -
I would little by little just start eating healthier, for example eat a banana instead of chocolate as a snack and looking at my diet and how I can improve. It doesnt have to be a drastic change. Just small steps so you can eventually build a good habit. Its good to take vitamin and mineral supplements too if you are lacking in anything but check with your doctor
also its good to make a habit of going to the gym - or if you cant afford it just practice with a video at home. theres lots of easy workouts on youtube to follow. I reccomend Chloe Ting
Also, perhaps try a new style or fashion trend you like. Or even your haircut/style. Theres many things you can learn about yourself trying new things and it will help you develop
Its difficult to say what is the perfect thing to do for mental health bc everyone is going through different things. But try to be patient with yourself if you are in tough situations. If you have poor mental health you can try to practice mindfulness, getting more exercise, trying a new hobby but things work differently for everyone. Taking vitamins and medicine can also play a part in your mental health, give you energy etc. So if you are taking it its important to do so daily.
Also, surround yourself with people who make you happy. Sometimes other ppls energy can affect how we act and feel, are you the best version of yourself when you are around them?
Sorry this is kinda long but I just wanted to share my thoughts ! good luck on your glow up<3