Put one hand on your throat and the other hand in front of your mouth. When you say a voiced sound you can feel a vibration in your vocal cords and you will not feel a push of air. If you say an unvoiced sound it is the opposite: there is no vibration in the vocal chords and there is a push of air from your mouth.
All vowel sounds are voiced, plus B, D, G, J, L, M, N, R, V, W, Y, and. The "th" in "that" is voiced, but the "th" in "thing" is unvoiced.
Here are some pairs of consonant sounds that have the same mouth position but one is voiced and the other unvoiced:
