Before I discovered the world of SAP, my background was the same as yours - Unix, Windows and Mac systems admin, with a bit of networking thrown in. I had also done some programming. The place I started in the SAP world was with Basis (called NetWeaver these days...). That's the area of SAP work that will most closely match your existing skill set.
There are many other areas within the world of SAP, obviously. You already mentioned you know about BI. Most of them, though, require business knowledge. That's not to say that a techie can't get that knowledge. On the contrary, I have. But I started in Basis and learned from there, and it took a while.
Of course, I know almost nothing about you, your abilities and your skills. Based purely on the few sentences you've written above, I'd suggest you start in an area that most closely matches your existing skills - Basis - and use that as a way to learn about other areas and see if any appeal to you.
But it is hard to make a good recommendation with so little knowledge. Bear that in mind when reading this and any other comments you may get.
Steve.