Contact Tracfone about transferring your service. Depending upon the phone model you may have to buy a SIM card (pretty inexpensive) or get an eSIM from them.
As for phone model, that is really a personal preference thing. I have an iPhone SE generation 1 which Apple stopped supporting several years ago. It does what I need so I just use it. Maybe next year I will find that old app versions no longer work (I am happy with old versions as long as they do what I need) and if I want to use certain services I need to update it. Personally I will look for an old model because those are the least expensive and I use them til they pretty much stop working completely, not when Apple says so. If you like staying on top of the curve, get something newer since it will last longer before in turn getting outdated.
"Senior" has no meaning in this. Many of the top level contributors here are "senior". We all have our own preferences and "senior" doesn't mean any kind of disability to use technology.