how to use itunes gift card to buy apps on ipad 2? Do I need a credit card for it?

No you don't need a credit card ro use a gift card if you already have an Apple ID. If you have an ID, go to the featured tab of the app store. Swipe to the bottom of the screen and tap on Redeem. You can enter the card number code in there.

You have to be signed into your Apple ID which is accessible in the lower left corner, if not already signed in.

I just redeemed Itune Gift Card $10 and I don't have Credi Card. I already redeemed in my ID, but I am unable to purchase Game Cash. Please tell me how to use it. I'm going to use it for purchasing Gem in Clash of Clan for $9.99.

I'm sorry, but there is nothing that I can do to help you with this situation. I don't know why, but many people have problems when they try to buy gems in the Clash of Clans game. Personally, I wouldn't touch the game with a ten foot pole, but that's just my opinion.

You will have to contact iTunes Store Support and ask for their help. You can email them from here.

https://getsupport.apple.com/Issues.action

I just redeemed Itune Gift Card $10 and I don't have Credi Card. I already redeemed in my ID, but I am unable to purchase Game Cash. Please tell me how to use it. I'm going to use it for purchasing Gem in Clash of Clan for $9.99.

Probably because $9.99 + tax is > $10.