the most important things that... you should know about those programs in order to perform succesfully?

The thing is Word and Excel are advanced Word Processor and Spread Sheet Respectively. So mastering them fully means mastering something Java, C or C++ (it may be bit exaggeration but conveys the point).

Minimum things you should know are
1.Excel
Inserting Graphs
Using Formulas

2.Word
Templates
Formating the text
Inserting tables, pictures

Other ppl may add to this list..

Depends upon the job and also what version they use. If you can do mail merge and some advanced things, and nice formating, and type medium to fast, and demonstrate that you have this knowledge, then you will probably get the job if that is what they are looking for. I would suspect that they are looking for many other skills also besides just word processing and excel. Like maybe people skills, friendly attitude, work ethic, etc.

With Excel that can be a lot of things. Excel is very powerful, and can do amazing things. Also, if you know how to print only certain parts of excel and fit to page would be good.

Get a copy of MS office and work with it for a week or two, push yourself to learn some new things. Both are feature rich.

Click here then click any of the results you see for help and tips with Word. Be careful, you don't have to buy and of these, there is so much free help on the web you don't have to purchase anything to learn this. Except maybe a good book from the local bookstore would help.

http://www.google.com/search?q=ms+word+tips+and+tricks&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-USfficial&client=firefox-a

Click here for excel:
http://www.google.com/search?hl=en&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hs=lz&q=ms+excel+help+tips+tricks&bt nG=Search