I play a 7-string bass. Some 8-string basses are tuned B, E, A, D, G, C, F, Bb or it could be tuned F#, B, E, A, D, G, C, F. It's up to you. My bass is tuned from F# to C. One difference between a 4-5 string bass and an 8-string bass is the width of the neck and the closeness of the strings. The bass is bigger and ususally heavier.
It takes a while to get use to all of those strings, but basically it is played the same way as a little baby 4 or 5-string is played :-)