In your letter, you asked [k4: tense inconsistency] ask me about my healthy lifestyle [k3: inaccurate wording - the friend doesn’t know if you lead a healthy lifestyle] whether I lead a healthy lifestyle and also about my attitude towards it.

As for your first question, of course, [k5] I do some things to stay healthy, such as doing sport exercises [k3: unnatural collocation] sports activities and eating healthy balanced food [k5: unnatural phrasing because of the absence of the comma] a healthy, balanced diet.

Answering your next question [k3: not very natural phrase structure], To answer your next question, I think  // Answering your next question, I'd say that observing a daily routine and taking care of themselves [k4: incorrect reflexive pronoun] oneself make  make / makes people healthy and strong.

Talking about a healthy lifestyle, I think that it is very important for everyone. So, every people [k3: incorrect plural form. "Every" means 'each one' and requires a singular noun/pronoun] everyone should do anything [k3: inaccurate word choice. 'Anything' means 'хоть что-нибудь'] everything they can to maintain a healthy lifestyle.

I'm curious about your coming summer holidays [k3: unnatural phrasing] upcoming summer holiday! Where are you going to spend your summer holidays [k3: unnatural for informal writing] break? What are you going to do on [k4: wrong preposition] during your summer holiday ? Who are you going to spend these [k4: wrong determiner] your summer break with?

Yours,

Mary