Please see some sight-singing resources below. Sight-singing is part of the ABRSM exams and a good way to practise reading music.
A few tips on sight-singing:
Check time-signature (how many beats in each bar?) and clap rhythm first before singing the phrase.
Check the key-signature (sharps or flats?) and figure out what step you are starting on.
Use either numbers (1-2-3-4-5....) or solfège (Do-re-mi-fa-sol-la-ti-do).
Sight-singing exercises: