What company should i activate my iphone with?

I have an iphone 4s i believe. I want to activate it but i don't know what company to activate it with. I'm not going to use At&t or other big company like them because i can't afford the service. I used to have metro PCS with an android, however, i talked to a metropcs worker about using my phone with them and he said they had to "flash it" he said this was jail breaking it which i don't want to do. I talk a lot as well as text, i used the internet probably less than 20 minutes a day -mainly to check email or school work. I was thinking of using H2O but i read that they have a lot of issues with the company. What i'm looking in a company is basically what i get from metroPCS but i'm wondering if Flashing my iphone will tweak it's software? I don't want to jail break it either. Which company can give me unlimited calling as-well as texting and some internet in case i have to look up things or homework assignments. I believe Cricket has a decent plan any feedback on that?

Straight Talk. $45 a month and runs off the AT&T old grid. Which is not bad. Also you get 3G which is not bad either. I tried the service before it was actually pretty good.

If it's an AT&T iPhone 4s, you MUST unlock it before you can use it on any company besides AT&T. And once you have done that, you can activate it ONLY on companies that use SIM cards (GSM).

If it is a Verizon or Sprint iPhone, then you can't unlock it, but you might be able to flash it. Flashing shouldn't affect the phone's software.