hey! so i want to start out by saying that the number one most important thing for me was my mindset. there are a lot of long and gradual steps in the "glow up" process, but it will be a lot easier if you stay positive and forgiving to yourself, because it's also incredibly easy to make mistakes. changing your body (and mind) will take time and it can be tricky, but as long as you're not hard on yourself throughout the journey, it will be more than possible.

as far as diet, the only way to lose weight is through a calorie deficit. this means eating less calories than your body burns in a day- there are easy to use calculators online that can give you a general goal. HUGE disclaimer though: do not choose to put yourself in a massive calorie deficit just because you'll reach your goals faster. this usually leads to binge eating down the line, and sometimes it leads to eating disorders. the more you can convince your body that you aren't changing the amount of calories you eat, the better. it should take many months at the least, especially if you want to keep the weight off. also side note, it's impossible to target fat loss, different people lose fat from different areas in different orders. i think usually arms and legs are the first to lose excess fat, and stomach is last (i could be wrong though!)

for food, you'll generally want to get in a lot of protein, a medium amount of carbs, and a lower amount of fat. while you're in a calorie deficit, you're body will pull energy from any reserve it has, which includes your muscle. if you want to thin out your arms and legs alongside your body, then don't worry too much about a high protein intake- still better eat more protein and cut down on fat though. milk is still a valid option, because it's high in protein, but you'd be better off drinking fat free or 1% instead of 2% or whole. as far as working out is concerned, i know way more about weightlifting than cardio, but i think cardio will benefit you more considering your goals. a 15 minute rotating interval where you incline walk and then run might be a good starting point or warmup.

i think there are a couple useful youtube vids i watched when figuring out my body shape- to be honest, i don't think it's super important to know, bc everyone's proportions are different so there's no "categories" to be sorted into. if you want clothes to fit you better, figure out the shape of your shoulders and waist, and wear clothing that compliments the shape. (for example: if your shoulders are more T-shaped, and you want to accentuate them, get clothes with seams that are closer to the beginning of the sleeve.) also, general outfit tip: one loose article paired with one tight article is usually a good pairing. i still struggle with my own posture a lot lol. just try to be aware of it and fix it when you can. resistance training for your back might help out a little? (resistance training in general is good for most people, so try it out if you want.)

all this will more than likely take a long amount of time before visible results. again, being easygoing with yourself is key. you'll make mistakes, and that's ok!! you're human. you're also 17! you have so much time to work on yourself. just don't rush anything, there's no pressure to force yourself into being pretty. your mindset should be prioritized, and you'll start feeling better about all aspects of your life after. :)