How and where can I catch Giratina and Cresselia and what pokeball should..

For both of these you need the national dex. For Girantina, go to the new area below Veilstone on route 214, and just keep going up until you find him. Use dusk balls. For Cressilia, go to the Sailor's house in Canalave city and find out that his son is in a coma thing. Go to the sailor, and head off to new moon island. Go up into the place, talk to Cressilia, and it runs. Get the item, give it to the sailor, and now you can hunt it down. Use a masterball if you have it.

Don't double post. Well, kind of double post... Refer to number 2 I left in your other question.

For Giratina, you can always use a Dusk Ball. For Cresselia, wait until it's night, get her stuck in battle, and start using Dusk Balls.

Do not ask a question twice... and use a Master Ball on Cresellia if you cant stop it and Giritina I caught with a Dusk Ball

Check top for giratinas where abouts. Dusk balls would do great(obviously) in caves which you probably already knew. Cresslia is harder to catch then mesprit. I read you should use a master ball on it. You could do whatever you want. I would do a pursuit rythem. You find it use pursuit, it takes damage it flees, reapeat till read, get a mean look Pokemon, and throw lots of balls. To find cresilla talk to mom and boy in sailor house, by boat in canalave. Then talk to sailor and he will take you o fullmoon island. There is only cressila who will run away like mesprit but leave behind the lunar wing which you give to the boy and he is fully recovered. Yay. Then use the marking map to find it. You can do an easier way of actually seeing cressila is to go to jublife enter the route exit the route reapeat till cresslia goes to your route. Go to where it is and bingo we have a winner. Hope I helped. Oh and for person at top look here for cresslia.
Hope I helped=)