In the beginning one played "bottleneck guitar" - and that was exactly what it was. You took the neck of a bottle, preferably one with a straight neck, and played with the neck put on a finger. Later people started to play with tubes made of metal, e.g. steel or brass, or with specially made glass tubes.
You can play with the guitar tuned for playing with a slide. Usually the guitar is tuned in an open chord, i.e. if you do not depress any string, the string are tuned in a chord. The most common ones are an E, A or G-chord. It is also possible to play with ordinary guitar tuning.
It is hard to say who was the first one to play slide, but the first big name, and the one who created his very special style, was Robert Johnson.A number of great blues artists have learnt a lot from him: Muddy Waters, Elmore James, Eric Clapton.
Later the slide has been used in other kinds of music as well. Among wellknown slide equilibrists we notice Ry Cooder, George Harrison, Lowell George, Bonny Raitt, Sonny Landreth and many others.