Unfortunately the only way to get good at this is to start with simple melodies (folk songs, nursery rhymes, etc) and build your skills by practising every day.
It is important that regardless of what you work on to develop your sight reading skills that you know what it should sound like before you start
An important thing to remember is that you are trying to do two or three things when sight reading:
- interpret rhythm
- interpret and play notes moving smoothly from note to note
- interpret and play chords moving smoothly from chord to chord
Some important precursors to study are
Scales
- both hands same direction up and down
- hands move in opposing directions - left down right up and reversed for second half of the scale
- hands move in opposing directions - left up right down and reversed for second half of the scale
Arpeggios
major, minor and dominant 7th chords to begin. Add augmented and diminished triads and then the other 7ths (minor 7th, major 7th, diminished 7th, etc)
Both of these (scales and arpeggios), should be read first, with tempos as slow as 30 bpm (1 note every 2 seconds if playing quarter notes/crotchets) and moving as fast as 240bpm (4 notes every second)
After doing some basic skill building with scales and arpeggios, reading melodies should improve simply because your mind recognises the notes and what position/fingering will work best to play segments of the melody.
This advice works for any instrument, as the problems and challenges are the same regardless of medium. The instruments that offer the most challenges, however, are the strings, as there are many possible positions for some notes that the instrument can play