First, if you haven't already... I'd recommend you read
the guide (
unabridged here).
Second, buy a used, carrier unlocked GSM phone now (
have a guide on how to shop used if you need one). Physically transfer your SIM card (if it'll fit - it might not due to being too small), otherwise obtain a new SIM card from AT&T for your new used phone and perform any necessary data transfer between handsets at this point. Do this before the 14 day window is gone.
Third, return the new phone to AT&T before the 14 day window closes and ensure that your contract is terminated.
Fourth, buy a SIM card from whatever MVNO you choose to switch to and proceed to port your number only at this point. I no longer recommend AT&T's Cricket Wireless for the same reasons I no longer recommend GoSmart Mobile (or technically Sprint's Boost Mobile which was never in the guide), details on why
starting here.
If you don't think you can manage a data diet sufficient enough to make any AT&T MVNOs work for you in alternative to Cricket's prices, review your usage and mull over
this. The reality is, you probably need far less data than you think, and it's really easy to cut back through deliberate, conscious usage.
Best of luck.