How would I got about shipping something out of state?

I bought my sister an iPhone case and I want to ship it to her, but she lives out of state. I've never really shipped anything so how would I got about doing that? Like do I need stamps or something or what?

The easiest thing is to just make sure you have your sister's full address, and then bring the iPhone to your nearest post office. They will be able to sell you some packaging and advise you on the different shipping options. For something so valuable, you'll probably want to use a shipping service that includes "tracking", to guarantee that your package reaches its destination.

For something that size, you should use Priority Mail and put it in a box, rather than a bubble envelope. You can get free boxes for Priority Mail at the post office. You can't easily use stamps on Priority Mail, so take it to the post office and pay there. Then you will get a receipt so you can follow it's progress online.