Sing on a note that is in the upper middle part of your range. For a soprano, C above middle C is a good place to start. Mezzo's should start on an A. Tenors and basses should start on the same pitches, an octave lower.
Vocalize the sound "who" on the first note and hold it for two beats. Then, sing down an octave and sing the sound "oh" and hold it for two beats. Finally, sing back up to the original pitch on the sound "oo." Continue to sing this series down a half-step each time until you can't sing any lower.
Find the pitch a third above the bridge in your voice. You can tell where the bridge falls because of the change in timbre and the sensation that the sound has changed location in your body. For instance, if you sing in head voice, you will feel the sound in your head. Those that sing in chest voice will feel it in their chest.
Using the word "hello," sing the first pitch on "hel" and then sing down a perfect fifth to the second pitch on "lo." This will help you practice going over the bridge in your voice and make your voice more reliable and consistent.