Hi Clare, welcome to the forums
The best way to get better at sight reading is to sight read! Just grab a bit of music, check the key/time signatures. have a look through for any accidentals, then choose an appropriate tempo and go for it. If you do one piece a day you'd be surprised at how quickly you improve! Also, if you search 'Sight reading' in the appropriate forum for your instrument you'll probably find lots more advice more specific to your instrument