Download it here: http://www.apple.com/itunes/download/.

This is how you download an app:
1) Go to the home page by pressing the home button.
2) Open the "App Store".
3) Find an app you want by looking around or searching.
4) If it is free, click the "Free" button and install the app.
5) You will need an Apple ID in order to download an app.

If you have anymore questions, just ask.

Thank you very much but there are somethings which I say:
I'm from Iran and though I paid for that iphone but probably can't make an apple ID unfortunately. what should I do so?
And, for seeing and choosing an app from "app store" icon on iphone, it should be connected to the Internet. yes?

And about that itunes that I'm downloading it from that link you gave me, what should I do with that? I should install it to my machine (Windows 7) and try to connect with it to my iphone by cable, yes?

Yeah, you connect your iPhone to the PC with a cable and sync songs and apps to it. You can do it all through the phone though.

Yes, you need internet for the app store.

The Apple ID is free. Sign up here.

Downloading itunes (11.1.1) from apple faces error, so I tried to download it from another website and could find two editions which were 32-bit and 64-bit. I know my iphone (5S) is 64-bit and also my machine (Windows 7) is 32-bit but don't know now which one of those two editions (32 or 64) I should download. I think I should download the 32-bit one because it runs on my machine. Is it correct?

Download the 32bit. What was the error?

OK. Only it was written that download has faced an error or something like that, a short message. So I removed it a tried to find the file from another websites.
Thanks mate . If I had any more problem about my phone I'll post it here again.

I connected the phone to the Internet using my home wifi. And then for installing a free app from app store of the phone I entered my Apple ID but now for installing those apps I should enter a visa or master card which of course it's not possible for an Iranian person in Iran to have an visa or master card account because of the bans. The only solution appears a gift card or a gift information as it's written on the phone as following:

"Billing Information"
"To create an Apple ID, you must provide credit card, gift card or gift certificate information. You will not be charged until you make a purchase."

Now what is the solution?

There is a way to make an Apple ID without putting in credit card info. I did it a few months ago for my grandpas iPad. This article should help: http://support.apple.com/kb/HT2534

JC713: Thank you very much dear mate. I successfully created an Apple ID installed some free apps because of your help. Thanks again.