3rd Gen quick leafgreen question

After beating the elite 4, can i get mewtwo right away? or do i need the national dex to get to him? i only have 61 pokemon caught...

I think you need to help Lorelei on Four Island first.

Get the Ruby in First Island, help Lorelei, find the saphhire in Six Island, beat the Rockets in Five Island and give the saphhire to Celio in First Island. Then you will be able to catch it.

go to the volcano on island 1 beat team rocket and go to ceo and he will give you the slvi pass to go to the sivli islands then in island 6 get the sapphire in the dotted hole.some one will take from you and you must go to rocket ware house in island 5 to get it back.

ok i get that but do i need the national dex or not?

no, that isn't needed to get mewtwo

Technically, you do need the National Dex if you want to get to the other Sevii Islands. You won't be able to open up the Cerulean Cave unless you complete the Sevii Islands sidequest.