Wow... my congratz bro... I'm very proud of you. Another asset to our glorious motherland.
1.what type of things(maths,languages & etc.) I got to Improve till then
English is essential since all subjects are taught in English. Though most people who enter to university are not conversant with it, they used to learn it over the years just as the others. But if you can learn English it would be definitely advantageous. Especially mathematical terms in English like integration, differentiation, perpendicular, partial differentiation, polynomials, etc... that you already know in Sinhala.
2.which University is best to go to my target(electronics plus Robotics) Peradeniya or Moratuwa
Since your interest is on to Robotics & Electronics, I suggest Moratuwa. I just heard from a colleague that they have introduced more research labs for the electronics department recently as well.
First advise: Make sure that you do the subjects in the first year seriously. From the first year results, they select you to engineering streams. You will have to get highest marks for electronics. Unlike in A/L, you will have to compete with the best students of the island. All guys are just bright as you. So a huge competition. (This is one reason that I didn't want to go there though I passed for E1

). If you learn the subjects well, you will be able to do whatever you want.
Second advise: Do all subjects with your highest capacity throughout the 4 years and try to get a class. With a class, it is easier for you to do higher studies abroad like PhDs, etc...
Third advise: Enjoy some free time while you learn English and do scientific experiments. Because when you enter to the university, you will have to 100% concentrate on studies again.
Magneto, who is my esteemed colleague will add his comments soon. He is a bright fellow from Moratuwa University (MORA

) that you can get more advises in future.
Good luck buddy !!!