Infinite Steps in Safari Zone.

This trick is very simple. Find a patch of grass in there. Stand in it and just tap the directional pad lightly just so you would turn without walking. You should encounter a pokemon and catch it if you would like.

Making Items more than 100!

You need the HM Fly and Surf to do this glitch. Put the item you want to duplicate in the sixth slot of your inventory.

Go/fly to Viridian. Talk to the old man that ask you if you're in a hurry at the beginning of the game. Say no. Watch him show you how to catch a Pokemon.

Now fly to Cinnabar Island and surf on the right edge of the island. You should be able to battle a weird Pokemon called MissingNo. It's appearance is kind of weird. Don't let it attack you or don't capture it or it will ruin your game! Just run or beat him with powerful quick attacks.

After that, look at your sixth item and there should be a weird symbol next to it. Don't worry, it only means that you have more than 99 of it.