EDIT: Posted at the same time. Yeah, Sorvius is right.

But I have a question: how can encrypt save file? because when I save my variable with iFileWriteOpen () and iFileValueWrite () Functions, any use can edit it with notepad and make all efforts are useless.

Yep, it's something I've thought about as well. The easiest step would be using another extension than .txt. You can make up your own extension, like '.savedata'. The files can still be opened using NotePad, but most players won't realise.
Another method would be saving the file as .bin or even .jpg. This will definitely confuse anyone trying to open the file.
You could also save the data along with a lot of rubbish to prevent anyone from finding the 'real' values.
Furthermore, you could for example save your strings in the ASCII code. The average 'hacker' won't understand it.
And finally, you could use some childhood code words. Simply add x to the ASCII number of every character.

This discussion has been here before. Search the forum.

what is Function can convert string files to ASCII code?

No function needed.

void Main()
{
   string temp = "Hello World!";
   int var1 = temp[0]; //Returns the code of the first character in the string.
   int var2 = temp[1]; //Returns the code of the second character in the string.
   //Etc.
}

You can consider a string as an array of integers. Each of the items in the array holds a number which corresponds with a code in the ASCII table. If your programming is not that advanced yet, you can just ignore this last bit. It doesn't matter much.

Here's an idea, make it work first and THEN try to add to it.  When you think big (like I do), you have to step back and get the thing to work first and then add to it and enhance it later on.  Encrypting your saves isn't priority right now, its getting the game published in some stage of development.  We (I) will help you as you go, but make something we can play first, and if we edit our save files, that's our business

Even so, in the case of a multiplayer game, any save data you had would be checked by the server.  So, even if you changed it locally, they would just get read from the server and update to what they were.  at least that's how casual games do it to prevent cheaters xD

Think about piracy.  Some of the most successful indie games get their exposure from pirates making their game available for free.  They may lose money on some downloads, but if they try to crack down on saving their precious game data (like what EA does), people complain and attack them for being too worried about what people will do with their product.  If I buy a car, I am free to drive it into a lake even if the manufacturer gives it state of the art safety features

I don't think you understand what I meant.
Of course the files would still be editable with notepad, however, the use would have no clue about what to change

Thanks Roberto but I still edit it with notepad. 

I mean Content my save file don't change and all variables displayed as before.

Tomorrow morning I might give an example.

Thanks.

Hey behdadsoft, I'd like to see what you have made so far, no matter how broken or unfinished it might be.

I busy make a commercial car game, and show some parts soon.