Can I update my iMac? And how do I do that???

My husband and I were recently given an older iMac. I am not familar with mac's at all and I am having some difficulty getting the computer connected to the internet. Went to best buy last weekend and the gentleman told me that I needed an apple air port express to connect. I purchased it and got it home to find out that it won't install becuase I am running Mac OS X Panter version 10.3. I have also since then discovered that this computer has the old PowerPC processess and not intel and only has 1.25 GB of memory (which isn't much right?).

I would like to upgrade this computer for my kids to use. I would like it to be able to connect to the internet, and be able to install programs on it for them to use for school papers, etc. I am worried that it is going to be too much for a "Free" computer. What would I have to do to upgrade, what do I need to upgrade it to? Any suggestions you can give would be greatly appreicated at this point.

Thank you,

Tisha