You are asking a lot. But OK. Hang on, this is going to be a rough ride.
From the beginning of the series: Lord Voledmort killed Harry's parents on his way to kill baby Harry. Harry is the chosen one, the one who will vanquish LV (Lord Voldemort), because of a prophesy. That is why LV wants to kill Harry.But Lily gave her life for Harry and LV could not kill him. LV melted away, but his soul did not leave the earth, because he had made horcroxes to hid part of his soul. In book 4 he gets his body back, and he still wants to kill Harry who is now a teenager at Hogwarts.
Snape was once a death eater and is now a secret agent or double agent, for, you are not suppose to know which side. There is a reason Snape is doing what he is doing. I don't thing you will know that until book/movie 7.
In the 6th book/movie. Snape has taken an unbreakable vow to help Draco. Draco has been given the assignment by LV to kill DD (Dumbledore), because with DD out of the way, LV can get to Harry.
DD is on a quest to find and destroy all of the horcuxes. He wants informaton to confirm his theory and to know how many of them there are. He needs the memory of Slughorn.
It is a dangerous time. Death Eaters are killing people and openly killing Muggles in the Muggle world. Dementors have joined the Death Eaters and LV... but more of that in book 7.
DD goes to the cave to get a horcrux, the locket. He has to drink the poision to get at it. There are inferni in the water guarding the locket.
Draco manages to get the DE into Hogwarts and then up to the tower.. I think you can understand the rest from there... except for the locket. Someone else got the locket first and put this other one there. That someone is RAB. If you remember the picture that Slughorn had where he was showing some of the Black family, that will be your clue. By the way, Bella, the crazy DE is Sirius's (who died in book/movie 5 and was Harry's Godfather) cousin. Bella is Draco's aunt, his mother's sister.
The couple you saw for just a short time in the house before the DE's came for Harry, are Tonks, an auror (wizard policeman) and her husband Remus Lupin, a former teacher at Hogwarts and a werewolf. And the hairy DE is a werewolf too, but the movie never tells you that. He is the one who bit Remus when Remus was a kid. Greyback is his name and he like to bit children. He wanted to bit DD on the tower in the book, but you don't see that in the movie.