This is a simple, 3 step process:
1. Get yoself an unlocked, GSM phone. I got a Motorola Moto X, but the Google Nexus 5 is another great option. There are choices aplenty on eBay as well. My Moto X cost $300, but on the other hand, I'm not bound to a 2 year contract! So the extra $100 or $200 you pay upfront is worth it. At least to me it is.
GSM is the technology T-Mo and AT&T are based on to where you just need a SIM card and an unlocked phone to swap providers. Verizon and Sprint are CDMA-based, where it ain't as easy as sticking in a card.
2. Get yoself a SIM card. I got mine from T-Mobile's prepaid service FO FREE (via a Slickdeals post) but one shouldn't cost more than say $10. AT&T Wireless prepaid is another option.
3. Get yo number ported to your new prepaid service. I easily did this when signing up for T-Mo on their website. It was part of the signup process. This will automatically cancel your account at your previous wireless carrier or at least that's what happened with me.
So I am currently on T-Mobile's pre-paid service with a sweet phone. My plan is 100 minutes (I rarely talk on the phone and at this point in my life, I'd rather text, email, or Facebook message), unlimited texting, and 5 gigs of data for $30 a month. Awesome.
WELCOME TO WIRELESS FREEDOM!!
And here is my brand new celly:
T-Mobile Prepaid - The vaunted $30 a month plan (100 minutes, Unlimited text, 5 GB data). This is where I got my SIM card FO FREE (via a Slickdeals post) but like I said, a new SIM shouldn't cost more than around $10.
Motorola Moto Maker - This is where I bought my Moto X for $300 via a Slickdeals post. At Moto Maker, you get to design your phone, which sold me. It's literally the reason why I picked the Moto X over the Nexus 5! I'm superficial like that. I'm very into aesthetics. My phone is Spearmint Green with Metallic Cherry as the accent. Yes, I'm a chick. Nexus 5 only comes in black, white, and red.. and my wallet is red!! See how vexed I am?
One Plus One and Republic Wireless - I've read good things about these companies that sell cool phones sans contracts but I'm not a customer of either.