The reason I'm writing to you
First of all, (comma) you shouldn't eat too much fast food, because then you are losing/ not getting a lot of vitamins which are very important for your good health.I know that this is very difficult for everyone, but that is something that you had better do.
Also, (comma) smoking
Moreover, (comma) why don't you go to the gym? I think that this is a great idea and if I were you, (comma) I would bring (along) a friend with me, too. You could also buy a bicycle so that you could go every day to the university with it and cycle every morning near the beach/ coast.
I hope that I have helped you with this advice
*Advice (and food)= uncountable noun --->