Trouble with iPhone 5s
Assuming that you have an iCloud backup since you tagged your question with the "iCloud" tag.

If your old iPhone was backed up via iCloud then you just start the Setup Guide (the thing you see when you first start up an new iPhone) and go through some basics like language and connecting it to your Wi-Fi network. After that you will be asked if you want to set up the phone as A new iPhone, From an iTunes backup or From an iCloud backup. Choose iCloud backup. In the following step you will need to enter your Apple-ID login credentials. After that your iPhone will start downloading the data and it will take care of everything by itself.

The Setup Guide is easy to follow. Just read carefully and follow the instructions.