If one forgot to open a can of apple juice before Shabbat how should one proceed on Shabbat itself to open the can? Is there a difference if the can would have held a solid e.g. tuna as opposed to a liquid?

It is forbidden to open the can the regular way you would do it during the week since opening the can would be creating a "kli"- a utensil. The problem is not in the content liquid or solid , rather in the can itself. The Shmirat Shabbat K'hilchata chapter 9, suggests in a case where one forgot to open the can before Shabbat to make a hole on the bottom of the can or in its side, thereby rendering the can useless to be used as a "kli. Once that is done, the can be opened normally, but obviously the contents will have to be placed in a different utensil.