I'm just gonna try to be specific , so if it isn't , don't blame me. At least I tried.
You should have beat Cynthia already and get the National dex as speed freak said.
Go to the Pokemon league , down the waterfall , and in victory road. Cross the bridge and use rock climb to get down ( I think , because I caught a Lapras before so yeah, ) . Go left then down or press the top part of your + . You will see stairs and go up, then you follow this path since the stairs only lead you 1 way. (Or so I think ) , then you see a cave and then enter it, use defog and if this is your 1st time then Marley will accompany you with her Arcanine . Marley is the girl with a black and white striped dress. Now you have to fight some trainers while going straight ( if this is your first time ) and then go right , ( not sure ) . Again I am not sure but maybe by this time you'll see a rectangle pond with water . Surf in it , and there is a chance you will find Lapras. I'm warning you though , Lapras has ice beam , brine , safeguard and perish song. When Lapras uses perish song, you better catch it or you'll have to look for another Lapras.
You should have the game boy advance game Pokemon Fire red / leaf green in the cartridge slot in your ds and you can look for a Growlithe at route 201 and route 202.
You should know or remember the way to mt. Coronet , and there is a waterfall on the way, Use the super rod in front of the waterfall ( don't go up it ) . And there you can find a dratini / dragonair.
You should have some honey , which you can get at floaroma town and use it on any honey tree. Honey trees ( aka tress that have yellow leaves ) can be found at route 205,206,207,208,209,210,211,212,213,214,215.218,221,222,Eterna forest , floaroma meadow , fuego iron works , which is under the bridge before the valley wind works , surf to the left and go to the land and a building . There are patches of grass there before the building , you can also find honey trees at the valley wind works.Wait for a few hours , and if there is shaking , you should find a munchlax . Munchlax evolves into a snorlax by happiness. You should try leveling it up , walking a lot , battling with it, giving it berries and other things like maybe entering contests with it can increase it's happiness . When you catch munchlax, use a luxury ball and give it a soothe bell , to help with the happiness. Also you can go to the resort area, but you'll need at least ten contest ribbons to enter it . Go to the 2nd floor and you can massage the Pokemon at the top left spot. This only works once a day though. In veilstone city, from the Pokemon center , go down and then go near the gym . The house nearest to the gym is the house of the massage girl . This only works once a day aswell. Munchlax should evolve into snorlax after you keep on doing these things . Plus you can get contest items from the massage girl in veilstone.