You did a great job writing that! I was so happy to read it all! Let's do a quick edit on the punctuation. 

Hi, I just want to say that if(1-conjunction) you are reading this, I know you are a great person(.) * You should end the sentence here. When(Capitalized W) you feel like the storm is coming,(2-Comma). You (Capitalized Y) just need to smile. One(Capitalized O) million people left to smile every day. Why?(Capitalized W) I don't know.

Smile, it's free. You don't need to pay. You only need to know that someone loves you, and that is enough. If you are reading this, I hope tomorrow you give a lot smiles for people that is waiting to have a great day. Sometimes we feel like nothing is changing. That we have a routine, and day after day,(*I may be wrong with this one. Somebody clears it, please). we do the same things. Do you know what do you know for change your routine ? ( I believe it will be better if you rearrange it this way: Do you know what do you do to change your routine?) Just smile.

Corrections1, and 2: When you use conjunctions in the beginning of a sentence, you have to use  a comma between the two sentences, and then write the second sentence after the comma. This will make you avoid a run-on sentence, or a fragment.

The run on sentence: It is when you merge two independent clauses without using the proper punctuation. Example: I love watching movies but I don't have time to watch them. 

Fragment: It is when the sentence in incomplete.

I hope this helps :)