How can you trade between ruby and crystal?

you can not trade crystel with ruby because Pokemon cristel is a different generaion than Pokemon rudy

You can't. There is no way to move second or first generation Pokemon to the third or fourth generations. This is because the data is stored completely differently, and conflicts among stats....

In firered, you must have obtained the ruby and sapphire from the sevii islands.

yes as long as you have a connection cord and there both from the same operating system such as game boy advance or color or so on then yes.

it would help if you mean fire red or plain red fire red you need the national pokedex regular red you dont