Do I have to sign up or be with t-mobile to get the unlocked iPhone 6?

I have to wait to sign up with a carrier bcuz of my age. I heard apple.com only sells their unlocked iPhone 6 through t-mobile but you can jus take out the SIM card and switch carriers. I'm wondering if its as simple as buying the unlocked iPhone 6 and using it for apps n stuff until I'm old enough to join a plan or do I have to be eligible to join a carrier just to order an unlocked iPhone 6 off of apple or amazon. Also I'm looking for ppl has done this or understands this fully

Yes, it's that simple. You can buy the phone online or at the Apple Store. It is designated as the "T-Mobile" edition, but it's actually just the unlocked GSM phone. Beware that the phone is GSM, so it will not work on CDMA carriers. In other words, it will not work on Sprint or Verizon, but will work on T-Mobile, AT&T and most other carriers.

Once you've bought the phone, you can get a prepaid SIM card from a company like Simple Mobile, NET10, or even T-Mobile. These don't require a contract… You just buy the SIM card online or from a store like Best Buy or Walmart, pop it into the unlocked iPhone, and add money to your account by buying phone cards.