I had the same problem, here is what I did to get round it (not sure why it worked but this is exactly what I did):
Save game near the sign at the bottom of cerulean saying TRAINERS TIPS.
Load a DIFFERENT VERSION OF THE ROM (the ghost should not be there, it's a glitch)
then load your old save game and the ghost should not stop you in this ROM.
(I couldn't open the door when I first tried, but I did a lap of the nugget bridge and saw a few Sprite glitches on the way round, then the door worked and the ghost wasn't there.)
Good luck.
Failing that just get a different version altogether and start again.
Cerulean city is in Kanto region. You can go through saffron city to get there. It is the north exit in saffron city!
Gym 2 is Cerulean City, and the Gym Leader is Misty ("The Waterflowers of Cerulean City" is the Pokemon episode with Misty and her three sisters). To get to Cerulean City, you need to go through Mount Moon.
After beating cerulean cities gym head down from cerulean and enter the underground path go through then go down and there is vermillion.
go through Mt. Moon. its left of Pewter city
You have to leave Viridian City and go through Mt Moon to get to Cerulean City. You may meet some bad guys on the way through Mt Moon and also get a fossil.
you cantYou can't get to Mt. Moon from Cerulean City in HG/SS, you have to go through Diglett's Cave and head east from Pewter City.
Mewtwo is in cerulean cave, and you can only get Mew through an event.
By walking there from Saffron City or through Rock Tunnel from Cerulean City.
Just beat the Cerulean gym -> Go to Bill's house -> Then the house that a police was standing in front the door would be unlocked and you can go through it to whatever route!
Beat the sevvi islands and the Pokemon league. Then go to Cerulean city and get to Cerulean cave. Once you're inside, navigate your way through the cave. Mewtwo is at level 70. Be sure that you are ready to give it all you got. ^_^
Go east(or right) of cerulean city(you'll need flash to get through the cave)
You have to go through the Mt. Moon, which is a cave located east of Pewter City.