Im just know gettng Pokemon diamond and I'm on the 5th gym leader and ..

The way you worded this was a little confusing, fyi. However, I believe I understand what you're asking. You need to obtain the first 4 badges. Then Cynthia will ask you to go back to route 214 to heal all those psyduck. Once you do that, she'll appear once again and give you the old charm which she wants you to deliver to her grandmother in Celestic Town. Keep following the route, make sure you have HM05 Defog from the Great Marsh/Safari Zone place, and use it on that route, and proceed to Celestic Town. You can look over the town if you want, but go down and talk to the old woman. Battle the galactic grunts, and she'll talk to you some more. Go inside the cavern and examine the painting on the back wall. She'll come and talk to you once again, and you'll give her the old charm then. And now, go to Hearthome City and go in the Pokemon Contest Hall and talk to the gym leader and your mom, surprisingly. The gym leader will then go back to the gym and you should now have access to the 5th gym leader.

And I'm not sure how to get the girl her suite key, either, but I'm guessing that you have to attend some Nintendo Event in order to get her suite key.

I noticed that I answered the first part to this question WRONG! Way wrong. Here's an updated version:

Talk to the Galactic Grunt when trying to leave pastoria city, and follow him to the Valor Lakefront. On the way, you'll battle your rival. Beat him, and continue on. Following the Grunt, you'll eventually battle him. Beat him, and go north a little and you'll notice Cynthia will talk to you. She'll give you a secretpotion to heal the Psyduck along route 210. It doesn't matter whether you fly to Veilstone or Solaceon now or later, however if you continue along route 214, you'll end up at Veilstone, so it doesn't matter. Either way, go to route 210, eventually or now, and heal the Psyduck. Cynthia shows up and gives you the old charm, which you'll need to give to her grandmother in Celestic Town, so that is obviously your next destination. Along rotue 210, you will have to use defog, which is obtained in the Safari Game in Pastoria City. Go to Pastoria City, and go in the bottom part, talking to the old woman along the way. Battle the galactic grunt, talk to the old woman again, give her the old charms, go in the cave, examine the drawing, talk to the old woman again, get hm03, and run off to hearthome city and battle the 5th gym leader.

The suite key is right above the place where you can heal your Pokemon it is on route 213 use the dowsing machine to find it