I understand that sharding is a solution...
There is no a common vision of what is sharding in the context of blockchain networks. Every project has its own vision of what is sharding, how sharding should be implemented and what set of problems it should solve.
Is sharding already implemented in Ethereum?
No, it's not. When someone says "it's implemented into the roadmap of the prospective transition", it means "it's not implemented".
Where can I find more information about it?
You can find a lot of information on the subject. However, you should keep in mind that this information is incomplete and not finalised.