Cailan

15 years old and ready to take advantage of your biggest advantage over us olld folk, time. Congratulations, you’ll do well.

Fat loss is only sustainable and effective if done over a long period. The quick fix systems are great for dropping weight while often not touching excess fat.

Basics to consider for calculations. Remembering these will help you keep motivation long term as things go slower than the junk advertising tells you is possible.
Fat is 3,500 calories a pound
Subsequently losing a pound of fat a week means a deficit of 500 calories per day, 2 pounds is 1,000 a day and as much as you should ever aim for.
Deficits above 1,000 a day can and often do send the body into starvation mode where it is far more likely to burn muscle for energy than fat.
Weight should only ever be used to assess trends, if you are doing this for appearance and you like the way you are looking that is more important especially because of the below.
Fat is light compared to a most other body tissue. Muscle weighs far more due to it’s water content and water fluctuations can easily skew weight.

An example of why people become disheartened is below, I am using your weight as a starting point for illistration. It is also to show why the scales shouldn’t rule you.

Diet and exercise program starts at week 1
End of week 1 shows a 4 pound loss to 14 st 10
For this to be fat would mean having burned 14,000 more calories than consumed
Actual daily deficit was 750 calories meaning 1.5 pound max fat loss
The body hasn’t go used to unlocking stores yet so 1 pound was fat, half pound was lean muscle catabolised 2.5 was water fluctuation.
End of week 2 sees a 1 pound gain to 14 stone 11
The calorie deficit was slightly lower due to missing a bit of training but still at 500 calorie a day average.
The body has got used to activity and burned the full pound of fat this should
The half pound of muscle has been rebuilt as the body realises it will need this and likely more in the future
Some of the water has balance has been redressed and the body is holding an extra 1.5 pounds of this
End of week 3 sees weight stable
Disappointed by the previous week the deficit is up to 1,000 calories a day
2 pounds of fat have been burned
Muscle mass has increased by half a pound due to increase of training, this speed of gain generally only occurs in the very early days
Water levels have swung slightly higher than original with another 1.5 pound gain.

Seeing nothing after the initial loss person gives up despite having gained half a pound of muscle, half a pound of water which could disappear the next week and losing 4 pounds of fat.

This is not a prediction just a possibility to remember. Reality is likely to be more of a downward trend.

I would advise aiming for 1.5 pounds fat loss a week, as this is a bit below the maximum realistic loss and therefore more achivable.
Your aim of 21 pounds by Christmas this year (8 weeks away) would mean 2.625 pounds a week loss which is not something you can safely aim for, especially as a begginer who is still growing. Sorry about that. However the 1.5 a week would mean 12 pound lost and have you close to 14 stones by Christmas and if you get closer to the 2 ponds max there is a safe chance of beating this.
The June targets are closer to the mark. 5 stone would need 2.1 pounds a week loss but the 4 stone would need 1.7 so is within safe realms, and not far off my 1.5 target which would give you close to 50 pounds fat loss as a 16th birthday present to yourself.
Remember the scales may put you over the weight the fat loss would give you due to muscle gain from the increased activity so judge based on appearance and clothing more than actual weight.

The system I am going to suggest may appear too simplistic but it has worked for everyone that has ever stuck to it. I call it the 10% diet.
I don’t know what you eat but unless you are consuming a ton of pure sugar or fat a day don’t cut anything out just cut down across the board to a ratio of 10%. Same food you are used to just a small amount less of it.
Increase what you do physically by 10%. For an olympic athlete this could be massive but for the average person this could be as little as walking up and down stairs 5 times a day. This is not boot camp it’s introduction to movement.
You will feel both, the drop in food will make you feel slightly hungry but nothing extreme and you will soon adjust and become used to the new level.
The activity increase may make you ache a bit, not a lot and it will soon fade.
Once you are used to this do the same again, it could take a couple of weeks and if you have been overeating to a large degree before you may not have hit calorie deficit yet, I do know one person who after 8 weeks realised it was the first time in years he hadn’t gained weight over an 8 week period but stayed stable.
Your body will be used to doing more so the increase could involve going for a walk, cycle or you may have come back here for an at home circuit routine. The change will make your body start craving different foods to provide sustainable energy during activity, starch products like pasta, potato, rice it will also call for sugars which you ideally need to ignore as much as you can. The overall drop still needs to be 10% unless you are already losing 2 pounds a week regularly.
As the last part suggests you keep cutting intake and increasing activity until you are consitantly losing 1.5 pounds a week of fat or more, which needs to be assessed on trend.

There are plenty of people here including myself who can advise workouts you can do at home when you need them. I haven’t posted any here because if your activity level is really low now you will be best suited to just taking a walk and getting used to moving first then starting a formal routine once you are used to this. If you are already relatively active let us know and we can give suggestions.
First and foremost will be enjoyment. You may know stuff you already enjoy, you don’t have to be good at them, that comes with time, if so tell us or start doing it. If you don’t try things out, if you like them stick with it if after a few weeks you hate it, move on to something else. I love running, weights and other random insanity so I do that. I can think of little worse than playing football every week or similar so I don’t do them, but others love it.

The 4-5 stone loss will likely take longer than you are wanting here but you can safely loose close to 3.5 of fat and have gained a bit of lean mass while doing so.
The next step will be to decide how you want to be at 17, 18 etc. Doing so will set you up for a lifetime of fitness and in 25 years you could be the fit old fogey helping someone else like you.