What do I do?

The key is not really important. What you must do is get your 6th gym badge. You must also talk to Cynthia's Grandmother in Celestic Town. After you have done this, you must have a conversation with Professor Rowan, Dawn/Lucas, and your rival in the Library. The Library can be found in Canalave City. After you have done this, you will be able to access Lake Valor. From there you must help Dawn at Lake Verity. Once you have done that, you must battle the 7th gym leader of Snowpoint City. You must then help your rival at Lake Acuity. This storyline will lead you to face Team Galatic on the Spear Pillar. After you have defeated them, you will go into the Distorted World. At the end of he Distorted World, you will face Cyrus and battle Giratina. After you have done all of this, you will be able to go to Sunyshore City.

Hope this helps!

what should i do to go to lake acuity but there is no way to get outside of mt.coronet and is said that you need rock climb but the rock clib hm is in route 217 please help me please

Alright so here's what you do:
To get into Lake Valor you have to have beaten the sixth gym in Canalave City. Your friend will then say to come with him. He will bring you to the Canalave Library. There will be Dawn/Lukas and Professor Rowan. Professor Rowan will talk about the Pokemon of the Lakes and say that you three will have to guard them from Team Galactic. Then there will be an earthquake coming from Mt. Coronet. Dawn/Lukas and Professor Rowan will guard Lake Verity and be cornered by Mars. Your friend will guard Lake Acuity and be cornered by Jupiter. You will guard Lake Valor and then you can go in.
To get into Sunyshore City you have to go to the Galactic building in Veilstone City and Looker will give you a tour of the place. Get the Master Ball which is in the door after the crowd gathers and goes away, but you have to talk to Cyrus to get it. Then go out and beat the seventh gym and save your friend from Jupiter. It will just be a small talk. No battling. Then save Dawn/Lukas and Professor Rowan from Mars. In this one you have to battle. Win and go into Mt. Coronet(oreburgh city side)and use Rock Climb to go up to the next room. Follow the path until you first get outside. Don't use Rock Climb just go left and there will be a door. Go through. There follow that path until you get outside with a choice of two Rock Climb paths. Go up the one on the left. There are three paths. You can take any one of them. Follow them and there should be a cave door. Go in. Follow that path. Then you should be at Spear Pillar. You will have to battle two Galactic guys then if you win you and your friend battle mars and Jupiter in a two-on-two battle. If you win you will get to see Cyrus and he will summon Dialga and Palkia and then Giratina appears mysteriousily and Cyrus gets sucked into the portal where Giratina vanished and you and Cynthia will go and find him. The four of you will be in the Distortion World. You have to follow the path and then solve the puzzle of the Pokemon of the Lakes where you have to push Uxie's boulder into the hole then Mespirit's then Azlef's. Then a path will form where Cynthia was blocking you. Go and follow the path. Cyrus is at the end of the path. Battle him. Then you will have to battle Giratina. Catch or defeat him. A portal will appear. Go inside. You will be at Sendoff Spring. Cynthia will be blocking a cave door. You need the National Dex to go in. Go back to Sandgem Town. Talk to Professor Rowan. Go back to the hotel. Now you can go to Sunyshore City.
To get the Suite Key you have to use the Dowsing Machine App to find it. Then bring it to the lady and she will give you a White Flute.