Are 당신 confused about the alternate time line? In the season 5 finale, when Juliet set off the bomb, it created two separate time lines, one where they appear to have been blasted back to the present, and one where the plane never even crashed on the island. If your seeing dead people on the island, it's probably Hurley's ability to see them. This only explains why you're seeing dead people, but the whole show can't be explained with one answer. 당신 need to read summaries of the seasons, 또는 better yet, watch the entire show in order to fully understand.
Either you've missed episodes out, 또는 I recommend 당신 watch an even simpler show. Either way, 당신 won't get the proper 로스트 experience 의해 asking everyone for answers. Watch the episodes and figure things out for yourself.