Check the first post. There are currently 22 games that are currently BC for preview members. If you have already purchased any of those digitally on the 360 then go to your "my games" page on the X1 and scroll ALL the way to the right past the games. There should be a page that says "ready to install". Your previously purchased digital 360 games will already be in there ready to download.
You can also download them from the
xbox.com site here. It will say "download to 360" but you will actually be downloading them to the X1.
If you have the physical disc, you just put it in and it will install to your HD. This goes very fast since most 360 games are fairly small in size. Then you just play it. Very easy.
If you are almost out of space then I would recommend re-visiting
this thread for an external HD when you have the extra money.