One of the best ways is to just try it, and see what questions come to mind. The
easiest way is to download the desktop version of Linux Mint (which is based on
Ubuntu, but has some things pre-installed that make life easier) and "burn the
image" to a CD. Then you can boot from the CD and try it out, without making any
changes to your hard drive.

When a question comes up, use Google to find the answer. There are so many
forums and blogs about Linux that you can find the answer 99.9% of the time,
without having to ask a question.

There are two types of hardware which might give you grief. One is wireless
network adapters, the other is printers. And to a lesser extent, webcams.

There are free, downloadable manuals and user guides, but I find most of them
have an overwhelming level of detail.