How to change an iPhone 4s from At&t carrier to T-Mobile?

I'm currently with T-Mobile and I have a Windows Phone, and I HATE my windows phone.

I have received an iPhone 4s from my grandma and she says I can have it, but the problem is that she used AT&T for the iPhone, and when I took the AT&T sim card out and replaced it with my T-Mobile sim card (There both the same size and fit), but the whole iPhone just restarted from the apple screen then it turned on, and when I was going through with the setup process and it connected to Wi-Fi, then it displayed a message saying SIM NOT VALID. THE SIM CARD THAT YOU CURRENTLY HAVE INSTALLED IN THIS IPHONE IS FROM A CARRIER THAT IS NOT SUPPORTED UNDER THE ACTIVATION POLICY THAT IS CURRENTLY ASSIGNED BY THE ACTIVATION SERVER. ITS NOT A HARDWARE ISSUE WITH THE IPHONE.

I don't want to spend money i'm saving up as much as I can and it costs $55 and I don't want to spend any money.

What can I do to get my iPhone to work from an AT&T to T-Mobile?

What is the best app for Smoothie Recipes?

I have a Vitimix and am looking for an Iphone app that would give me recipes for smoothies alcoholic and healthy ones as well as possibly other Recipes for my blender.

Any suggestions? Free is obviously preferred but I'm open to all.

Is it possible for Whatsapp to switch between online and off if not using the app?

For about a week I hadn't received any Whatsapp msgs from my boyfriend but when I logged on I'd notice sometimes his status showed him "online" then sometimes not. He said if he wasn't chatting with me he wasn't using the app, that with an iPhone you stay logged into Whatsapp unless you manually close it, and if he's connected to the internet it will show him online even if he's not actively using it. Is this true? Will he show online when conntected to the internet on iPhone, or only when he's actively using the app?

Unlocked iPhone not working with other sims?

So, my iPhone 5 which was bought from carphone warehouse and they state that all their phones are unlocked and they confirmed mine was unlocked. On my holiday to india, I tried many different networks and it says 'sim not valid' what's wrong here? Is carphone warehouse just lying and is there a way to get around to it next time I'm abroad?

How to delete messages on iphone?

On my phone it says messages are taking up 7.8 gb of data? I went into my messages app and deleted all of my messages and it still says that? And it says my iCloud storage is full? I don't even have my pictures on it. I'm so done with this apple bullcrap.

How to sync songs to my iphone?

I bought some songs on my computer and a lot later on laptop - I managed somehow to transfer all the songs from my laptop to my computer and they're all in the library on the computer. But only the songs from my laptop will sync now? All the songs show up from the library but none of the ones from my computer will sync - there's a button thing which this 'on this iphone' on itunes where it lists only the ones that will sync - any idea how I get everything from that library to sync to my iphone?

How to turn off iPhone 4s flashlight?

I drop orange juice inside my bag and my phone got wet 3 months ago and the flashlight won't turn off. But after 1 week it turn off. Then suddenly yesterday it turn on after a long time. What should I do?