Is it difficult to learn drawing on graphic tablets?

Its not really difficult to learn how to sketch on a Graphic tablet but keep in mind that it is much better to sketch on paper first, scan the outline of your sketch and then post-process / color your outlines using Photoshop / CorelDraw or any other tool of your choice.

just need practice if you have already a talent in drawing..!

Point is this that u know drawing basics already or not.
Use of tablet is exception.

Learning how to use the tablet is not difficult.
For any tool, it largely depends on your willingness to learn how to use the tool.

It is nice idea to practice drawing using graphic tablets, but also it is enjoyable to use the classic method in drawing, plane paper and sharp pencil is the most attractive tools for an artist to start drawing, learn how to draw.
Good Luck Salma :)