do it the easy way. first ( you dont have to be logged in or anything) press on "free" on an free app and then chose the register button. now when you can select your payment method there should be a "none" option aviable. this way you dont have to have a credit card and can only download free apps or just use a gift card.
so again
1. open itunes
2. make sure you are not signed in with anyone else's account. i.e you are logged out
3.pick a free app
4. hit register when it asks you for your username
5. enter valid information
6. choose "none" in the payment method section