Do these apps apply data charges if i use Wi-Fi?

I have 2g data plan and i have an iphone 4s and i watch youtube and go on instagram and play games a lot but all the apps i've downloaded are free but if i watch youtube videos, go on instagram, and play games on Wi-Fi, will the data charges still go up? Or if you use Wi-Fi, does nothing get the data to increase?

How to make a Verizon iPhone 4s work with AT&T?

My friend gave me his old iPhone 4s, but i have AT&T i went to AT&T and they gave me the sim card but thats it. How do i activate this phone to work with AT&T? Please be very specific.

Added (1). Andrew- that didn't work!

If i just got an AT&T contract can i still switch to straight talk?

I just ordered the iphone 5 from AT&T and then discovered straight talk and was wondering if i could switch to it even though i'm in contract. If so please let me know how to go about doing this. Will i still have to pay AT&T if i do switch to straight talk? Will i get my deposit back that i gave to AT&T when i signed up? Please let me know all details! I really want the unlimited data from straight talk that i won't get with AT&T!