Ok, so I have straight talk I'd like to buy a new or different phone how do I know what will work? 

So there is a pretty easy way to know as every phone now has a SIM card SIM Stands for Subscriber Identity Module carriers use this SIM Number to keep track of you its like a fingerprint    when you take your SIM out of your phone or E-Sims for those with Newer Phones 

You will see the numbers start with 89148 Verizon Towers, 89012 T-Mobile Towers, 89014 AT&T Towers

Hopefully this information will be useful to you and help you understand better what kinds of phones you need