A bass tuner allows you to hear the proper pitch for each string. If you play the tuner while tightening and plucking a string, eventually you will hear the pitches match. You've found the proper tuning. The Internet can lead you to free digital tuners to practice with or you can purchase one from a music store.
Pressing the fifth fret of the E string creates an A note. Pressing the fifth fret of the A string creates a D note, and the D string's fifth fret produces a G. If you begin by fretting the E string and creating an A note, you can tune the A string to that pitch. Tighten and pluck the string until the tone it makes matches the tone of your fretted string. If you repeat this for each string, you will tune your bass to itself.
If you touch a string instead of pressing it, your bass produces a harmonic note. By using the fifth and seventh frets of strings, you can tune your bass. Create harmonic notes with the A string's seventh fret and the E string's fifth. Pluck them at the same time; if the sounds match, your A string is in tune. Tune your D string with its seventh fret and the fifth fret of the A string. Tune the G string with its seventh fret and the fifth fret of the D.