Dear Linh! 

Thank you for your asking for some advice to keep fit and healthy.

These are my ideas :

-Firstly, do some exercises, like example: running, jogging, swimming,...

-Secondly, eat healthy food

-Thirdly, play fewest computer game as you can

-Finally, spend more time outdoors than watching TV at home

I hope with my advice, you will fitter and healthier.

Love,

Phuong