Question: I am a Trinity grade 5 pianist and have been playing piano for 2-3 years(earlier played keyboard).

I find very difficult to sight read. I usually play the passage once very carefully and memorize it well so I don't have look at the music while playing. I play while continuously looking down at the keys and find it difficult to lift my head up and register the notes and then play. I know all the notes on the staff, but I can't register them in time to play them on the spot.

What should I do to get better at sight reading?

Answer: 
As with anything, it will improve with practice. And, as with anything, *you should tailor your practice to the problem you're having*.

Since your problem is looking down at the keys, devise a practice method that prevents you from doing so. My teacher growing up always held a notebook above my hands so that I couldn't look at them. Do you have someone in your household or school that would be willing to do that for the 10 minutes a day you practice your sightreading? If not, you could always try rigging something to blog your view. (Or you could really be ridiculous and get one of [these](https://rads.stackoverflow.com/amzn/click/B076VYRT1B).)

Once you've mastered this, you may eventually have a problem with not looking ahead while you sightread; to address this, have someone cover up the current chord you're on, which forces you to look ahead. And so on.

See also recent responses to [What is a good way to practice piano sight reading?](https://music.stackexchange.com/questions/65879/what-is-a-good-way-to-practice-piano-sight-reading)
Sight read more! The best way to get better at doing something is to practice it. Force yourself to keep your eyes glued to the page. Play slow, deliberately, and correct your mistakes.