After you beat the Pokemon league what else do you do?

After you beat the Pokemon League, go to Professor Oak (if you have 60+ Pokemon in your Pokedex). He will talk to you and give you the National Dex, which will let you catch all 386 Pokemon. (From the Kanto/Hoenn region)

You have to beat the Pokemon League to move the guy blocking the cave.

I'm not quite sure where to find those, just keep exploring! ^_^

You need to turn the Ruby and Sapphire over to Celio before you can access the Cerulean Cave. The Mushrooms are attached to Paras and Parasect.

You can complete your whole Pokedex (Good luck)...
To get the guy to move, you need to get the Ruby from Mt. Ember on island one, and find the Sapphire in the Dotted Hole on island 5, then bring them to Celio which will establish connection between The Ruby/Sapphire games and your Pokemon Fire Red.
I find my mushrooms by going to the Safari Zone and catching as many Paras/Parasect as I can. Most of them should have tinymushrooms but some Parasect will have bigmushrooms...

After you have done the Pokemon league get your Master Ball and go to the cave and the man is gone so go in and catch a Mewtwo!

Use theif on a Paras or catch him.

After you beat the Elite Four you could comeplete your Pokedex. You get the guy at Cerulean Cave to move by beating the Elite Four and completing the quests on the Sevi Islands 4-7. You can find tiny and big mushrooms on Paras' and Parasect's.

mushrooms are found when you catch paris and paricect there nholding the item

You can get the three legendary dogs in fire red according to Starter Pokemon

Charmander=Suicune

Squirtle=riaku

Bulbasaur=Entei

If you have 60+Pokemon you should go to proffeser oak he will give national dex

You can find tiny or big mushrooms in mt moon just catch a paras it would be holding a tiny mushrooms

To find a tiny or big mushroom, you must have a Pokemon that is able to learn the Thief move then go to Ceruelean Cave or Mt Moon and then if you saw a wild Parasect use the move Thief. Thre migt be a cance that you may get a Tin or Big Mushroom. I wis thi could help you!😁

After you beat the Pokemon league you just go to One Island help professsor and get the guy out

To enter the cave you have to go to the One Island and go to the Mt. Ember and find for Ruby (it is in the the cave behind Team Rocket). Then go on Six Island (you will receive the ticket when you bring the ruby to the cellio) and open the door on Ruin Valley with Cut. After go on Five Island on Team Rocket Warehouse and beat the team rocket and will earn the Sapphire (bring it to cellio) And finally the cave is unlocked!