Beginner Q2: Installing software

[Thomas Blasejewicz]

I do have great difficulties installing software!
When I tried Synaptic, I get for each and every software package I tried
the error message:
"Package operation failed. The installation or removal of a software
package failed."

AND there are quite a number of software packages I actually downloaded
before, but which Synaptic apparently does not know (the search function
does not find them, although they are already right here on my computer)

Yet, a guide book (quoted below) says, you should NOT install anything
manually:
Ubuntu Pocket Guide and Reference
Keir Thomas
page 124

"Manually installing a package
It is inadvisable you attempt to manually download a package and install
it. The process is fraught with problems, the most significant being the
potential difficulty in manually sourcing dependencies. "

So, what can I do to avoid the above mentioned error messages
AND
install software that Synaptic does not recognize
(a search within Synaptic for example shows, that it does not find
"Flash Player", which is definitely available in a version for Ubuntu.

Since I installed Ubuntu, I was not yet able to install ANY software -
and the guidebooks and documentation did not help me so far.