Can I make an iPhone for another carrier work for T-Mobile?

I'm thinking of buying my brother and I iPhones, but I don't have AT&T, Verizon, or Sprint. I have T-Mobile. I know nothing about this, so my question might be a little vague, but what does it mean when you might not get 3g speeds? I know you're supposed to jailbreak for it to work. Also, where would I buy it from (should I buy from Best Buy or the actual carrier store)? I checked on the Apple website, and for the unlocked, it was $549.00. Would I have to buy one from one of the provided carriers, or buy the unlocked one?

I'm thinking of buying my brother and I iPhones, but I don't have AT&T, Verizon, or Sprint. I have T-Mobile. I know nothing about this, so my question might be a little vague, but what does it mean when you might not get 3g speeds?
--- GSM carreirs (carriers athat use sim card) have 4 radio frequencies they use for 3G. Its called UMTS. Normally its 850mhx, 900mhz, 1900mhz, 2100mhz, like ATT has. But Tmobile is different with 3G. They only use 1700 mhz. And 2100mhz. Tho only thing in common from ATT and Tmobile phones is the 2100mhz 3G band. This means that any ATT or Othe GSM phone on Tmobile will have spotty 3G coverage. Why? A cell tower on any carrier DOES NOT give off all 4 2G bands and all 4 3G bands. That requires ALOT of power. Instead, each tower may only give off 1-2 2G signals and 1-2 3G signals… So, in your case, what if your local tower ONLY gives off 1700mhz! Iphone radios can't get 1700 bands. Therefore, since the ipohone can't get 1700, and there's no 2100 being given off… = no 3G for you…

I know you're supposed to jailbreak for it to work.
--- jailbreak it… Go to Cydia app and unlock the iphone, and iinstall the tmobile flashing setup app.

Also, where would I buy it from (should I buy from Best Buy or the actual carrier store)?
If you do get an Iphone, and you want it on Tmobile, do NOT get it at a carrier store. Part of the iphone deal, you must get service with them or no Iphone sale for you… Instead, if you want it new, go to an Applestore. If you want it used, go to swappa.com.

I checked on the Apple website, and for the unlocked, it was $549.00. Would I have to buy one from one of the provided carriers, or buy the unlocked one?
---dont go to a carrier store… Just get the unlocked one

My suggestion to you would be to buy a locked ATT iphone from ebay or on bulletin boards and get it unlocked by a factory unlock service like iPhoneUnlockedCheap.com. This is what a lot of people do. I don't recommend jailbreaking because if done incorrectly it could render your iPhone inoperable. Factory unlocks do not touch your iPhone. I don't suggest buying a locked phone from any other provider than ATT because to unlock it is outrageously expensive.