The Lighthouse is in Olivine City.
level up your pokemon, and try as hard as you can to avoid some people
there isn't a pool
Ask the pharmasist in cianwood city to give you one then give it to the lighthouse Pokemon
You get it at the Cianwood City Pharmacy. It is used to Cure The Lighthouse Pokemon Ampharos. This answer is right-on. the pharmacy in Cianwood City in the Johto Region, and it is used to cure the ailling lighthouse Pokemon ampharos.
Get the secretpotion from a guy there and go back to Olivine in the lighthouse and cure the Ampharos in lighthouse and then battle Jasmine
talk to jasmine in the lighthouse at olivine city and then go to cianwood and talk to the pharmasist
Take the elevator at the bottom of the Lighthouse
You find it in the Glitter Lighthouse in Olivine, or in the Celadon Dept. Store.
You must go get some medicine for her from cainwood, and bring it to her at the top of the lighthouse. Then she will return to her gym.
Olivine city.
You have to go in the lighthouse and get the gym leader. Then you fight the gym leader and the guy in front of the town will leave and you will be able to get there.
There is no best Pokemon on Pokemon HeartGold.