Welcome.
You could save yourself a few dollars by searching on Youtube for Michael New.
He has a whole host of free video tutorials on sight reading, music theory and practical exercises.
Once you have the basics of music theory, note durations, notation, repeats, key signatures etc and can understand the notation terms like dal segno al fine then its practice, practice, practice etc.
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Try the following:-
Pick up some new sheet music,
Look at it,
Study it in your head,
Play the notes in your head,
Think about what notes you are going to play with what finger,
Take time doing the above,
Play the song on your keyboard at a slow tempo,
Play it again,
Play it again.
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Now take a break, do something different, then pick up some new sheet music and repeat.
Do not repeat the exercise using the same sheet music, each time you do the exercise use new music.
Constant repeating the same piece of music will get you playing it faster but you are just learning the song and muscle memory comes into play.
Its the performing the exercise on a new piece of music that will get you to sight read better and faster.
Practice, practice, practice........ that means its now time for me to turn on my Keyboard and get on with my first 15 minute practice session of my day.
Good luck.