Sightreading is an important skill that will allow you to be confident in many different musical settings. Good for you to want to improve this skill!
Before sightreading a new piece of music, check out the time signature. Be sure you know how many beats per measure, whether you are in simple or compound time. Also check out any tempo markings.
Next, notice the key you are playing in. If you have studied your scales, you should be able to play in a variety of keys. After determining the key, scan the music for accidentals.
Try to audiate (sing the music in your head) before you play it. Take a deep breath and go for it.
Good luck with your sightreading!