8 string ukuleles are tuned similar to Hawai'ian ukes but the strings are doubled in 4 pairs. The standard tuning is: GG CC EE AA. The other difference is the C and E strings are tuned an octave higher than Hawai'ian ukes. You can tune your uke using an online tuner like the one above or you can just tune it the pacific way - with your bro's uke/guitar or by ear: Place your finger on the 5th fret of the G strings, The open C strings should be tuned to that. Then place your finger over the 4th fret of the C strings to tune the open E strings. Place finger on 2nd fret of G strings to tune the open A strings. If your uke still sounds like a horror movie soundtrack after tuning, it might be a good idea to either buy or download a multi-instrument tuner App. You can also get Chromatic tuners which detect the pitch of each string you play and help you get it tuned accurately. Its normal for ukes to get out of tune, depending on how hard you play, changes in weather, newness/quality of the strings, if you use it to paddle your canoe, etc... Generally, the better quality the strings and pegs, the less tuning involved.
Alternative Tunings: Apart from the standard GCEA tuning, you can also tune your uke up another half or one and a half keys for for higher sound.
A# D# G C Tuning:
G# C# F A# Tuning (1/2 step up):