What do I need to tune a Double bass?

The lowest string is the lowest E on the piano. Next comes the A string which tuned to the A 1/4 above the E. Next 1/4 up is the D string and the top string is a G 1/4 above the D.

This is exactly same as the bass guitar and an octave lower than the 4 lower strings on a guitar.

The problem with tuning the double bass by ear is that it is very difficult for most people to distinguish the very low frequencies. I tune my bass using harmonics beginning at the 440 A , but that's a little advanced for you.

Nowadays many pro bassists use electronic tuners which will tell you whether you are exactly in tune or not. I suggest you get one of those AND a bass teacher.