You need to go to t-mobile.com and sign up there. You need the ICCD number for the sim card inside your new iPhone 6. Just go to Settings>General>About. It will ask you for your current carrier's account number/password, your device IMEI (not important), the phone number you want ported, etc. Make sure you select the pre-paid $30 100 minutes/5gb Data option. Simple, takes about 5 minutes.
If I'm getting this as a new line, instead of porting a number, will I get a new one?