How do I download free apps without a credit card?

Creating an iTunes Store, App Store, iBookstore, and Mac App Store account without a credit card

Step 3 is important, no matter whether you do this on a Mac or an iPad / iPhone:

Important: Before proceeding to the next step, you must download and install a free application. ...

Important: Before proceeding to the next step, you must download and install the free application by tapping Free followed by tapping Install App. ...

To summarize, you must first download a free app from the App Store. When you are asked to sign in with your Apple ID, select "Create new account". Accept the terms and conditions checkbox, then click Continue. After you enter all the requested personal data, click Continue.

That's all there is to it.

i have tried your instructions but its still the same. it ask for a credit card. they're not even accepting my debit card. Is there any other way to avoid it?

Buy an iTunes Gift Card and use that as your form of payment. If the app is free your gift card will not be charged.

Payment methods you can use in the iTunes Store, Mac App Store, App Store, and iBooks Store - Apple Support