Could you correct me, please! I don´t know whether I use well the modals or not!

Dear friend,

I am writing to you regarding to you for your problem to sleep well.

You should try to sleep only when you are sleepy. If you go the bed, and you can´t sleep, you should stand up and, I think you must** read a book or a magazine, or you must** listen to classical music, and you should do it on a comfortable seat. It´s to relax you!
You should try not to drink drinks which could put make you nervious. In addition, don´t eat too much dinner too much.
You must procure that when you will be on your bed, you have to be good.
Go you to a gym!. With the exercise you will feel tired.
I try to do all these things to sleep better, and they help me!

Sleeping is very important in the life, because if you sleep badly, you will feel tired all the day.
I hope that these things could help you to sleep.

I look forward your answer!

**In this sentence, "must" is correct, but "should" is better, I think. "Must" is when you have no choice ("I must drink water to stay alive!") but "should" is better for giving advice or recommending ("people should eat healthy food, not unhealthy food"). So, for this letter of advice, "should" is perfect!

This is great, Reich! Don't worry toooooo much about modals - they are very difficult, and even native speakers sometimes don't use the best one! ;)

Could you correct me, please! I don´t know whether I use well the modals or not!

Dear friend,

I am regarding to you for your problem to sleep well.

Firts of all, you should try to sleep only when you feel sleepy. If you go to the bed but you can´t sleep, you should/ can stand up and, I think you must read several page of a book or whatever things you like or you must can listen to a gentle song, which will make you really comfortable and sleep easier  and you do it on a comfortable seat. It´s to relax you!

Next, you try  should not to drink some kinds of drinks/ beverages which contain caffeinate, this can be harmful with your nervous and cause difficulty sleeping could put you nervious. In addition/ also, at the dinner,  don´t  dinner too much.

You must procure that when you will be on your bed, you have to be good. (I don't understand what do u mean hear)
Go you to a gym and With doing the exercise, which will make you feel tired; as a result, you also can sleep easier.

In fact, I myself try to do all these (above) things to sleep easier and better, and they help me a lots!!!

Sleeping is very important to our health.  in the life This is because if you sleep badly, you will feel tired all next the day.

I hope that these things could help you to sleep easier and have a beauty sleep.

I look forward your answer!

Could you correct me, please! I don´t know whether I use well the modals or not!

Dear friend,

I am regarding to you for your problem to sleep well. 

You should try to sleep only when you are sleepy. If you go the bed (no coma) and you can´t sleep, you should stand up and (no coma) I think you must read a book or a magazine, or you must listen (without to) classical music, and you do it on a comfortable place. It´s for your relaxing! 
You try not to drink drinks which could make you nervous (without i). In addition, don´t eat too much before going to the bed.
You must procure (??? maybe convince) that when you will be on your bed, you have to be good. 
Go you to a gym! With the exercise you feel tired. 
I try to do all these things to sleep better, and they help me!

Sleeping is very important in the life, because If you sleep bad, you feel tired all the day.
I hope that these things could help you to sleep.

I look forward to your answer!

Could you correct me, please! I don´t know whether I am using the grammar correctly!

Dear friend,

In regards to having a good nights sleep.

You should try to sleep only when you are sleepy. If you go the bed, and you can´t sleep, you should get up and I think you must read a book or a magazine, or you must listen to classical music whilst in sitting in a comfortable seat/chair, it will relax you!
You should try not to drink drinks which could make you nervious. In addition, don´t eat too much, or eat at a late hour.
You should not worry about things when you go to bed.
Go you to a gym!. With the exercise you feel tired. OR doing exercise will make you tired.
I try to do all these things to sleep better, and they help me!

Sleeping is very important in the life, because If you sleep bad, you feel tired all the day.
I hope that these things could help you to sleep.

I look forward your answer!

When I cant sleep I think about how to conjugate Spanish verbs.

Could you correct me, please! I don´t know whether I use the modals well or not!

Dear friend,

This is how I regard your sleeping problem.

Like all verbs with an '-ing', 'regarding' is treated as an adjective or noun. You should say either "I regard..." or "I am regarding...". The object of 'regard' is what you are considering, so this sentence needs to be rearranged anyway. You could say "This is how I regard your inability to sleep well." You could also just say "sleeping problem" (here '-ing' makes 'sleep' an adjective describing 'problem') instead of "inability to sleep well".

You should try to sleep only when you are sleepy. If you go the bed, and you can´t sleep, you should stand up and, I think you must read a book or a magazine, or you must listen to classical music, and you must/should do it on a comfortable seat. It´s to relax you!

You've overused commas a little bit. 

"If you...can't sleep" is one clause, so no comma.

"I think..." implies that while you believe what follows, you also to some degree doubt it. It's usually used for giving opinions because it allows others to disagree. In addition, "I think you should..." is much more common than "I think you must..." for this reason.

It's much easier to say "...you should stand up and read a book or a magasine or listen to classical music on a comfortable seat." All of that falls under a single 'should'.

You should try not to drink drinks which could make you nervious. In addition, don´t eat too much dinner too much.
Go you to a gym!. With the exercise you will feel tired.
I try to do all these things to sleep better, and they help me!

'Dinner' is a noun. You could say "don't eat dinner too much"; but this is the same as "don't eat dinner too often". Since dinner is only eaten once a day, you probably want to talk about the quantity of dinner rather than the frequency ie. 'too much dinner'.

"You must procure that when you will be on your bed, you have to be good." I don't understand what you mean. What is 'that'?

Sleeping is very important in the life, because if you sleep badly, you feel tired all the day.
I hope that these things could help you to sleep.

'Bad' is an adjective. Adding '-ly' to an adjective makes it an adverb, which modifies a verb (whereas an adjective only modifies a noun).

"I hope that these things will help you to sleep." is better. "I hope..." already indicates that the possibility is not a certainty. It's even better if 'that' and 'to' are removed: "I hope these things will help you sleep."

I look forward your answer!

I hope that helped.