how can I print a selected area only?

"Sheet"? Do you mean a sheet of paper that is in your printer's tray? If the picture is smaller than the sheet of paper, it will only take up part of the sheet. All the other areas of the paper will remain white. The picture will be centered, unless your printer's software has some other options. If you want the picture off-center, you can add the picture to a document, such as Pages, change the Inspector Picture tab to "floating", and drag it to the position you want.

Much depends on the program, the printer, sometimes the OS, and sometimes, there is a box you can check, "Print selection only." Having to change my word processing app when I updated the Mac OS from Snow Leopard to El Capitan made that option unavailable when I used that word processor. With some other apps, I could print a selection, but often, I could not.

But all else aside, you can copy the selection, open a new word processing document, and paste the selection you copied into it. Print the document.

I hope this addresses your question.

Using what application and what printer?