Hi Malak,

Ideally the best way to approach this is to take a step back and look at the general game of rugby then to isolate the specific movements that include and rely on your core strength and stability.

As you havent specified what position you'll play i.e. Backs or forwards, it will be hard to explain the benefits and also to include specific exercises to improve your game, however a general over view should be ample as i'm assuming you're not playing professional (apologies if you are!!!).

The primary action and most common cause of injury due to lack of core strength and stability is the twisting movement as i'm sure you can appreciate that the core supports your spine many a trapped nerve or even a slipped disc can occur from poor and incorrect training methods.

So, first few exercises, although very simple will pay dividends.

Standing upright, feet shoulder width apart, keeping your hips and head locked forward(ideally with a broom across your shoulders holding either end like in a crucifix shape) facing forward twist 90 degress left the return to front then twist 90 degrees right and return to front. This completes 1 rep. It is imperative that this is done in a slow controlled manner to reduce risk of injury. Complete 30 reps for 5 sets. Or alternatively work to failure this will really burn.
Same exercise can be done sitting down, this reduces movement in the hips isolating your obliques and abs, but ensure good posture at all times!

Similar to the exercise above again in the standing position with feet just over shoulder width apart twist and bend trying to touch opposite knee so left hand to right knee (still with broom) Dont force though, however that little stretch at the end of the movement also helps improve flexibility but do not over do it!!!!

Hope these are enough to go on with. Have gone into some detail any questions or further execises required please dont hesitate to ask. (Please include position played)

Good Luck