if i want to create a basic sharepoint site. do i need a sharepoint designer ? or some other sharepoint software ?
I believe you'd just need the WSS 3.0 piece; which is free but which needs a "Server" OS. Won't run on XP/Vista.
To make complex sites: you'd need a programming environment like VS with the Designer plugins/addins to create web parts etc. To use advanced features like document integration, search, etc. you'd need to buy the MOSS 07 piece of Sharepoint.
Depends what you meant by create*. If you want to customize site look and feel then you need to use SharePoint designer. If you just need to create a site you don't need to use any tools or other specific software. By default SharePoint has builtin support to create sites.
Just start with SharePoint. Don't do anything with SharePoint Designer or Visual Studio until you understand what SharePoint really is and what it can do. SharePoint has a ton of features, and it takes some time to understand its strengths and weaknesses.