I am a UI/UX designer with only some screens of landing pages. I want to know if that will effective to get clients, if I build up some full imaginaries design projects from scratch?
This is a bloated question. UX covers many disciplines, UI Design/Site Hierarchy seems to be what your looking for but in this case just apply as a UI Designer.
If you want to learn about what UX is about check out Nielsen Norman Group and give a few great UX books a read, I recommend the following:
Reading these should really open your mind to what UX is. After this understanding you'll see visual Designs are just a way of communicating your Strategy.
If I was hiring a UX designer I would ask them to provide a case study for a relevant product. This would show me what disciplines and methodologies they understand and also show how they problem solve and focus on user centric design.
UX Portfolio Course
There is a great tutorial at Udemy that shows how to build a case study: