Will I lose weight effectively if I do this?

So I want to lose as much fat as possible.
I plan to swim for 5 days a week (breastroke 400 cals burned)
And eat only 1200 calories per day
And drink lots of water
I'm a 15 year old boy btw.
Is there anything else I can do?
Update: I also plan to run around my block for 20 minutes 5-6 days a week
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