esata to onboard sata port?

You have two options. The best way is to buy/install an eSATA controller card in a PCI slot. A real eSATA controller will include all the necessary functions, whereas a regular SATA port might, or it might not. Two of the really important things that are always part of eSATA is support for hot swapping and support for longer signal cables to reach an external device.

However, many plain SATA controllers actually do support these things. And many users don't really plan to use things like hot swapping. So the much simpler and cheaper solution is an adapter plate. These days they usually come with the external case. It mounts in a rear slot (one normally used for a PCI card) and has a short cord to run internally to the standard SATA port. On the outside it gives you an eSATA connector. Note that this is different - you cannot plug an eSATA connector into a regular SATA port. By the way, some earlier external cases came with an adapter cable instead of the plate system. It simply had a normal SATA connector on one end (for the interior port) and an eSATA socket on the other. You just had to figure how to route it through the case wall.