Tuning the Bass
Basses are tuned by turning the pegs the strings are wrapped around. Tightening the string raises the pitch; loosening the string lowers the pitch.
Many basses also have fine tuners mounted on each string near the tailpiece. With these the tension of the string can be adjusted more gradually by turning the small screw.
Basses are tuned E-A-D-G, with E being the lowest string.
The easiest way to tune a bass is with an inexpensive digital tuner. Many not only create a note to tune to but also have a display that indicates which direction the string needs to be tuned and when it's tuned correctly.
An alternative method is to use a pitch pipe, piano, or other instrument to create the pitches to tune to.
Experienced players sometimes use a tuning fork. An A-440 tuning fork creates the pitch for the A string. From that reference point a bassist tunes each of the other strings, recognizing the correct intervals between them.