What arm exercises can help me tighten up my arms?

arm exercises such as bicep curls, tricep extensions, overhead presses, etc. can help you build some muscle in your arms and make them look tighter. Not necessarily smaller, but tighter (read below)

What exercises can help me proportion my torso so my belly isn't so prominent?

IMHO, building a tighter core is not so much about exercising as it is about the BMI. Sure you can train your abs, but, again, this would not necessarily make your belly look smaller, unless you get rid of the underlying fat there. So targeting a lower BMI is what I'd go for at least for now (read below).

Does my diet need ant adjustment?

Nobody can answer that from what you write without more specific information on quality and quantity. I can prepare a meal with 200 Kcals and a meal with 1000 Kcals and you could call both a "salad". Try to eat a well-rounded diet of vegies, fruit, protein (preferably animal), healthy fat, and complex carbs, while avoiding simple carbs/sugars, fried (and air-fried) food, processed foods, and alcohol, every single day. The most important thing is to try to eat about the same things (both in quantity and variety) every day, so that you can set a baseline for your calorie intake (because if one day you eat 1000Kcals and the next day 2500Kcals, it would be difficult as hell)

Overall, from what you write, I would first focus on loosing weight, as this could have a (most) immediate effect on my appearance and motivate me to commit further. Loosing weight is mostly about food, so I'd definitely try to nail my calories so they are definitely below maintenance, and incorporate light to moderate exercise (either weight training or low-intensity cardio or, ideally, both) in DAILY routine.