What should I focus on to get admission in a Masters programme in the US?
Please keep in mind that only a small percentage of Master students get funded.

It's less competitive to get into a Master program than a PhD, mostly because the students pay their own way.

If your grades are not the best you should try to get high GRE, good reccomendation letters, and possibly to some research during your Bachelor.

Are any particular reasons why you chose the US? There are many other school around the world that would cost you less and get a good education as well.