I want to know where and how do I can improve my sketching skill..
You can do more practice during your free time . make rough sketches . try different mediums of sketching e.g. shading pencil,crayon,charcoal pencil etc
practice with pencil and also refer some basic sketching books initially. to know the line and curves
at a very much basic scale.. you need to learn and try to make sketch of very simple things.. like table, chair, then.. move up to the birds, animals.. do more practice on legs, mouth of animals. further you may try to sketch of diffrent parts of human beings. and do practice on hands, legs, nose, ear, whole style of face.
you can do it in free time after study. when you feel tired u can do . :)
. Go draw something, Repeat.Look at drawings, Draw from drawings,Draw from photographs,Be intentional,take classes , draw from life. Practice will improve sketching skills.