Normally from what I know there a few things that are given due importance or priority in a group discussion which is also a mass elimination round while selecting candidates for jobs!

Primarily ones communication skills, decision making skills, analysis and decision making skills would be tested and also graded. So you need to concentrate on those. I guess self confidence and your attitude too counts and he way you carry yourself! I feel that you could attend a few classes ! 

You have to improve your confidence. Attending a personality development class is the best way to improve your confidence and will power. Now companies are conducting group discussion after written test and person interview. When you get job in an organization, you need to work with number of people. Group discussion is the best way to get know about your interpersonal skill.  Through group discussion they can evaluate that how good you are at communication with other people. 

You are confident and good in English, but I think you lack practice. Practice makes you more confident and will give an exposure to speak on the spot to others on different subjects. I suggest you join some short term courses on ‘’Effective Public Speaking” or “Personality development”.  There you will get opportunities for group discussion that will be very helpful while facing the real situation. Be confident and go ahead. You can!

It is only problem of confidence. Some time we have knowledge to topic but can't speak single word. Discuss on some topics with your friends. 

The ability to express one's own opinion without fear and hesitation, makes a big difference.In a group discussion, these traits of fearlessness in a candidate are keenly observed by the organizers.

During the course of group discussion, all you have to do is focus and present yourself as an assertive, yet empathetic person. During these rounds, most candidates usually become aggressive and start pushing their own point of views rather loudly which creates negative impression. You need to be listening attentively and gauge others' point of views and use them to present your thoughts. Just keep cool, think on the facts and present them with confidence and clarity. All the best.

Hard selling oneself , boastfulness and oozing over confidence has an immediate negative impact anywhere , more so in group discussions. As long as one remains confident and communicative things will be ok. Although you feel a bit nervous in the beginning ,once you overcome it and get into the discussion things will seem much easier !

Giving space to others and hearing patiently with cool attitude, group discussions can be made lively. Confidence to express one's views without fear is another point that one should bear in mind in group discussions.

It is very easy for all of us to say that 'be confident', but we all say this to encourage you and hope that you become successful. Now, the point is that only our encouragement cannot make you confident. What you need is the inner belief, the faith in yourself can definitely give you the push you require. While you prepare for the group discussion, kindly say to yourself over and over again ,'I can do it' and see the magic. This will encourage you and your fear will take a back seat. If during group discussion you are asked to start the discussion or randomly you are asked to speak on the topic then never avoid it. Grab the opportunity instantly and speak your mind with clarity but never be aggressive and this will reflect your leadership quality which is of utmost importance. You can stand in front of a mirror and discuss any topic loudly, counter yourself and put forward as many viewpoints you can. It will also help you and if you get a chance to join any crash course on Personality Development as others have suggested will take you a long way but remember trust in yourself is the basic key to success in public speaking.    

Yes, it's easy to say "be confident". Encouragement from others acts as the first step for starting the process of being confident. And it is not easy to acquire a strong confidence overnight. It's a long process involving tons of hard work. Such people who are confident will never hesitate to express their opinions without fear or hesitation. They will not  offend others. If one is confident about his inner strengths,  Success will not evade them in group discussions.