How do I filter the sentences starting with a specific key word and then concatenate with the sentences which contains that keyword

You're not likely to get someone just giving you the code for this.
You need to try to solve it yourself and then as specific questions when you have difficulties.
If you don't know the basics of servicenow and javascript, first go find some tutorials and work through them. No one here is going to tutor you on these.
After you have the basic knowledge, write out what steps you would take if processing it by hand, in plain English (or your native language) and then translate that to the appropriate code.
Feel free to come back if/when you have difficulties.