Will my iphone still work when I go to the usa?

Going on hols and I don't know if my phone will work over there do I need to change it?

I don't see why it wouldn't. Though I'd advise you to put it in Airplane mode and keep it there to avoid roaming charges.

Could put back on Wi-Fi after it is enabled to have net though

It'll be fine, just bring a travel adapter in case you need to charge it.

Ya. It will work fine in USA