How to install a .tar.gz (or .tar.bz2) file?
I have downloaded indicator-multiload-o.2.tar.gz in my downloads folder.Tell me how to install
If you just want to install indicator-multiload you have done it wrong. You have downloaded the source code of the program, while you can use it to install it, you need to compile it, and this can be non-trivial for someone without programming training.
To install software in Ubuntu you should always look first in the Ubuntu Software Centre.
It the program is not available in it, or you want a newer version you should look for PPAs.
If there is no PPA avaliable or you want to just download one file, look for a .deb file. You should be able to install it just with a double-click.
There are more ways to install programs in Ubuntu, but if you are a novice you should probably avoid them for now.
Open the archive, and read the included README and/or INSTALL files. If there aren't any, there should be some other file which specifies how to install it.