A few generic tips:

- Don't think of it as a diet, do something that could theoretically sustainable for the rest of your life

- Learn about food and how it affects your body (that's an important knowledge investment, given you'll be eating for the rest of your life; nutrition is a complex field and studies are not super reliable, but you start to notice patterns with time)

- Try food replacement powders (or bars) to replace unhealthy meals with something more balanced (I use Huel but depending on geography)

- Eat enough proteins (how much depends on your goals and your body) - with some exercise, this will help you build muscle

- Sleep plenty

- Split your meals (and your proteins) across the day to avoid hunger

- Get a reliable scale and weight yourself at the same time every day

- Start with an online calculator (like iifym.com), try to eat that amount of calories for 2 weeks and monitor your weight

- Keep track of calories / proteins / fibres / sugar / saturated fat / salt in the food you're eating so as to reach your goals

- Do strength training to build muscle instead of tons of cardio (this will help you to build muscle, which will increase your metabolism) - I would still do some HIIT cardio for your heart's health

- When you reach a plateau and you're not losing weight anymore I'd recommend adding more cardio instead of cutting calories (mainly not to lower the amount of protein intake)

- Once you're happy with your body fat percentage I'd go back to strength training + little cardio.

On training:

Body-weight training is perfectly fine even if you can't lift weight (even though if you have health concerns you may want to consult your doctor, as body weight exercise can still be problematic).

This can be a good starting point: http://www.startbodyweight.com/p/start-bodyweight-basic-rout...

Once things get easy you can decide if you want to improve your endurance (by going for sets with high reps) or your strength (by going for sets with low reps and adding resistance - like weights).

My story (hoping you'll find something useful):

I recently lost 16kg from 84kg to 68kg over 4-5 months going from roughly 24% to 13% body fat. I may have lost some muscle but very little - my strength and my measurements seem to be increased.

What I do exactly on top of following the above tips:

- I'm on Huel 95% of the meals and "cheating" every once and then when I'm missing chewable food or for social eating (Huel helps because you know exactly how much you're eating)

- When eating food which isn't Huel I'm roughly calculating how many calories would that be and how many proteins - and then I'm adjusting my Huel meals or replacing some with protein powder

- I eat every 2 hours, 6 meals a day, from 10am to 8pm (fasting for 14 hours, which is something I've accidentally been doing all my life).

- For the first 3 months of my journey I did only strength training (push-ups, pull-ups, squats, various core exercises)

- In the last 2 months I added 1 hour of cardio per day as I wasn't losing weight anymore and I still wanted to burn more fat.

Future plans:

I plan to lose some more weight and then focus on building muscle + little cardio by going 200 calories over maintenance and cutting every once and then.

I already tried in the past to build muscle with 600 calories over maintenance and I definitely put too much fat on.

My workout regime is going to stay the same and I'll keep on eating mainly Huel (unless I create / find a tool which will help me create a diet as structured and precise as Huel and deliver the ingredients to my door).

Best of luck on your journey!

I'm not affiliated to any of the products / websites mentioned.