list prywatny 2

Dear Claire,

How are you? I hope you are fine. Thank you for your last letter.

You wrote me that you put on weight and you want be slim. I am glad that you asked me fot advice. I will try to help you. Do not worry, certainly you lose the weight.

First you need go to a doctor and do blood tests. You have to drink mineral water, because it will clean organism of harmful substances.

You should eat fruits and many vegetables. Do not eat many sweets, because contain a lot of sugar. You must runonce a day. If you can, You will ride a bike to school. This will improve your condition.

I promise that i will support you and if you want I write to you other tips.
Please notify me about your progress.

Lots of love,
XYZ

You wrote me that you put on weight 'and' (to zdanie niema sensu, daj 'but' (ale) you want (cos brak) be slim. I am glad that you asked me 'fot' (ortog) advice.
Do not worry, certainly (tutaj można dodac, 'with my help') you (cos brak) lose the weight.

First you need go to a doctor and 'do' (nie, to jest za bardzo potocne, lepiej..have blood tests done) blood tests. You have to drink mineral water, because it will clean (ale czyj? trzeba dodac) organism of harmful substances.

You should eat 'fruits' (l. poj) and many vegetables. Do not eat many sweets, because (cos brak) contain a lot of sugar.
If you can, 'You' (dlaczego duza litera?) 'will' (zle slowo) ride a bike to school.
I promise that 'i' (dlaczego mala litera, jak wszyscy pisza duza?) will support you and if you want I (cos brak) write to you (cos brak) other tips.
Please 'notify' (za wysokie slowo, tutaj ..let me know) me about your progress.

Why an article before weight?

you'll lose the weight = you'll lose your weight

I'd let it pass here