If English is not your mother tongue, you may need to make a clearer difference between the two sounds [v] and [w].
These sounds have some similarities. Both of these sounds involve the front part of the mouth. Both sounds are voiced: the vocal cords vibrate at the same time that you say each sound.
Now, here are the differences:
[v] is a fricative.
[w] is a semivowel.
Use a mirror to practice saying these sounds. Look for the position of the lips, and listen for the presence or absence of friction. You can practice making the difference between word pairs like these:
veil-whale vein-wane vest-west vent-went