First of all, we need more information about your device:
- what Android version?
- Stock ROM or custom ROM?
First thing you'll need is a custom recovery (TWRP recommended). Since Android Lollipop, Xposed can only being installed by flashing it via recovery.
For Samsung stock ROM (TouchWiz-based), you will need a special version of Xposed, this one from
here. Alternatively, you could use topjohnwu's systemless Xposed from
here if you like.
If you are running a custom ROM such as CM (which is not based on TouchWiz), you can use the official Xposed from
here (Android Lollipop) or
here (Android Marshmallow).
Installation instructions are given in each thread, so in case you have further questions, please ask for help in the corresponding tread.