To get better at sight reading you must:
> Read new music every day.
> The new music you read every day should be slightly lower than your level, so you can train your reaction and reading skills.
> Learn to read the musical notation (tempo, clefs, key signatures, accidentals) as if they were the signs on the road. If you miss one, you must stop and correct yourself.
> Your goal is to play with 100% accuracy in notes, and 100% accuracy in rhythm, on the 1st time you read it through.
Here are some good resources with exercises to improve your sight reading:
1) Czerny Op. 299 - learn 1 new exercises per week. These start out easy, but get harder towards the end of the book.
2) Four Star Sight Reading Series - I did all 10 books for 10 years of my piano career. These books give you 1 new exercise to play for each day of the week, as well as clapping rhythm exercises to develop your rhythm abilities. There are also ear training exercises for you to do at the end of each chapter.
https://www.amazon.ca/s?k=four+star+sight+reading
3) Download your favorite sheet music.
Sight reading ability also improves with how many pieces of music you have read and learned by reading sheet music.
Naturally, the more pieces you learn from sheet music, the faster you get at sight reading. The more difficult the pieces you read, the easier most sheet music will become to you.
YouTube is a good resource.
Lots of creators upload their sheets onto music-notes.com for purchase: https://www.musicnotes.com/
Musescore is also good: http://musescore.com/
I recently got Musescore Pro, and can find many arrangements made by users.
4) Memorize the 30 most common notes
If you don't already have the notes memorized, I highly suggest using flash cards to memorize all 30 common notes. If you know 30 people, you can match 30 names to 30 faces, then you can definitely memorize 30 notes on the grand staff.
You can also get an app to test instant recognition of notes and rests, or you can play this game: http://sightreading.training/
5) If you're tired of reading 1 note at a time, and dream of sight reading at 2 to 4 notes at a time, and you never had any formal training, here's a Free Course I made to help you review the basic fundamentals: http://easypianohacks.com/courses/sight-reading"