Hi Yasser,
What you’re asking is definitely possible.
How to start… by reading a lot! 
I would start by going trough the tutorial. So you understand the basic concepts of OpenHAB.
The next step is to install and set up the KNX binding.
Some interesting links can be found here and here.
Before thinking about controlling your TV, make sure that the KNX binding well. Create an item that allows you to toggle one of your KNX lights (that is how I’ve started).
The next step is to install the binding for your TV. You didn’t specify what TV you have, but you can find a list of bindings here. Depending on the make/model of your TV, you need to follow different steps.
When your TV binding is set up, you might need to create a rule to connect your KNX action with your TV. Some reading material can be found here.
Don’t be overambitious, take it step by step.