Need help installing files (I read the stickies)

[Arie]

HI, I have a Mac and am running OS 10.2.8
Im not having any problems downloading, however Im having a lot of difficulty installing the programs I am trying to upload.
For instance, in trying to install Adobe Photoshop CS2 or just CS, Ive downloaded several different ones, in RAR file format, and also in SIT format.
However I cannot either unzip them, or unstuff them. For the ones I have managed to unstuff, it is unstuffed in a "non recognizable format".
I am so confused.
I have downloaded a program here once before in the past without any difficulty, however cant remember what type file I installed then to get a working program.
Anybody who could be so kind as to possibly letting me know what types to look to download, or give me any idea what Im doing wrong, Id really appreciate it.Right now Im bogging my system down with filesharing for a bunch of files Ive downloaded but cant manage to get installed. :-(
Thanks in advance
Arie, a newbie
PS
Please be very specific and give step by step directions if possible. I am not really knowledgeable in the world of file sharing :-)