You could try reading up on it here:
The answers to your questions aren't all that difficult but it is
necessary to give them some thought. (1) Being is uncreated
. Melissus' argument is that anything which is created must a having a begining, i.e.
it has no previous existence prior to its creation. But as Being, by defiition, is eternal, it cannot have been created, (A corollary to this is that if it had no beginnning, it can ahve no end. Thus the eternal aspect is also demonstrated here. You may argue against this, of course, but the logic is sound.)
(2)If Being is uncreated (had no 'beginning'), it can have no end; ergo
it is permanaent (eternal) and unchangeable.
Everything else follows from these premises. Read the stuff that the links will take you to. Holler if anything seems unclear. We're here to help.